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Difference Between Plant Mitosis and Animal Mitosis

The cell division is the process by which organisms generate new cells. By this process, a single parent cell divides and creates identical daughter cells. During cell division, the parent cell duplicates its genetic material (DNA) and transmits to the daughter cells. 

There are three types of cell division such as amitosis, mitosis and meiosis. In this case, amitosis occurs in the lower animals like bacteria while mitosis occurs in the body or somatic cells and meiosis in germ cells producing sperm or egg cells.

Mitosis is a vital cell division process that occurs in the body repeatedly where one cell divides and produces two identical daughter cells. In this case, both daughter cells contain the same number of chromosomes or the same amount of genetic materials.

Animal mitosis and plant mitosis are the reproductive nuclear divisions which occur in animals and plants, respectively. During the mitosis, the newly produced cells contain the same number of genetic materials; as a result, the number of cells increases in the body which are crucial for growth, repair and regeneration.

There are four significant steps in mitosis such as prophase, metaphase, anaphase and telophase. The mitotic spindle occurs in both animal and plant mitosis. In animal, mitotic spindle occurs with the support of two centrioles, but in plants, it happens through without the assistance of any centrioles due to lack of centrioles.

During the cytokinesis, animal cells form furrow cleavage and finally produces two daughter cells. In contrast, plant cells contain a rigid cell wall that doesn’t create furrows, but it forms a cell plate at the centre of the cells that separate the two cell components.

Besides, all the time and everywhere, animal mitosis occurs while the plant mitosis occurs in the meristem tissues.

The following table shows some significant difference between plant mitosis and animal mitosis: 

Plant Mitosis

Animal Mitosis

The cells do not change the shape before cell division.

Before cell division, the animal cells become rounded.

Generally, mitosis occurs in the region of meristem tissues of the plant body. These meristem tissues are located at the tips of shoots, roots, and in the stem, between the xylem and phloem.

Animal mitosis occurs all the time and everywhere.

Plant mitosis is influenced by the plant hormone, cytokinin

Animal mitosis is controlled by the number of hormones but the functions of specific hormones are not yet known. 

Mitotic spindle occurs through without support of centrioles.

Mitotic spindle occurs in animal mitosis with the support of two centrioles.

Cell plate occurs at the center of the cells.

No cell plate occurs in the middle of the cells.

In plants, centrioles, centrosomes or aster are not found.

Centrioles, centrosomes, and asters are found.

Migration of centrioles does not occur.

Migration of centrioles occurs.

In this case, solid, middle lamella occurs between the two daughter cells.

Furrow cleavage occurs between two daughter cells.

The phragmoplast enlarges which is made of actin, myosin, and microtubules and makes the cell wall in plants.

In the telophase stage, a myosin and actin made contractile ring is present to make the two daughter cells.

During cytokinesis, mid-body may be formed.

There is no mid body.

There are no major functions of microfilaments in cytokinesis.

Microfilaments engage in cytokinesis.

Spindle persists as phagmoplasts during cytokinesis.

Spindle degenerates during cytokines.

The spindle is an astral type.

In this case, the spindle is amphiastral type.

Mitotic apparatus does not consist of asters.

Mitotic apparatus consists of asters.

image of Cytokiness in plant mitosis

Cytokinesis in Plant Mitosis

image of Cytokiness in animal mitosis

Cytokinesis in Animal Mitosis

Significance of Mitosis

Mitosis involves cells division and makes new cells. This cell division process duplicates non-sex cells which is essential for growth and development.

  • It helps to maintain proper cell size.
  • It is responsible for the development and growth of organisms.
  • It helps to distribute of chromosome equally to the newly produced daughter cell.
  • It helps to replace the old and dead cells in the animal body.
  • It maintains the balance of genetic material in the cells.
  • It provides a definite shape of the organisms.
  • It helps in the asexual reproduction of plants.
  • It helps to maintain the purity of types.
  • It also helps to do vegetative propagation in plants.

Meiosis: Definition, Types, Stages and Significance

Dividing cell is one of the important events in our life which divide each day, each hour, each seconds. In this process, a single cell divides to form two cells and again two cells produce four cells and so on. This process is known as cell division or cell reproduction.  

The cells of the particular species have a constant number of chromosomes. For example, human contains 46 or 23 pairs chromosomes (44+XY in male and 44+XX in female).  In sexually reproducing organism`s male and female gametes fuse together to form the zygote. If the gamete has the same number (46) of chromosomes as the somatic cells then the zygote would have twice (92) the diploid number of chromosomes. This number would go on doubling with each generation. However, the chromosome number always remains constant from generation to generation i.e., 46. This is due to meiotic division which reduces the chromosomes number to half and counteracts the effect of fertilization.

The procedure happening during gamete or spore formation and involving a reduction division whereby every daughter cell gets one of each pair of chromosomes, thus lessening the number of chromosomes in every cell to one half is known as meiosis. It is also called a reduction division. German biologist Oscar Hertwig first time described the meiosis in sea urchin eggs in 1876. But Belgian zoologist Edouard Van Beneden again described the meiosis in  Ascaris (roundworm) eggs at the chromosome level

In the process of meiosis, the chromosomes divide once and the nucleus and cytoplasm divide twice. Due to the meiosis, four haploid cells are formed from the single diploid cell. A cell undergoing meiosis is sometimes called meiocytes.

Types of Meiosis

There are three types of meiosis occurs in different organisms:

  • 1. Terminal or gametic meiosis (diplotonic pattern)
  • 2. Intermediary or Sporic Meiosis(deplohaplotonic pattern)
  • 3. Initial or Zygotic Meiosis (Haplotonic Pattern)

Terminal or gametic meiosis (diplotonic pattern): This type of meiosis is found in animals and in a few lower plants. It occurs just before the formation of gametes. The gametes are produced in gonads like testis and ovaries by the process called spermatogenesis and oogenesis, respectively.

Intermediary or Sporic Meiosis(deplohaplotonic pattern): It is the characteristic of higher plants and some thalophytes but is not found in animals. It takes place sometime between fertilization and the formation of gametes. In such cases, meiosis is followed by mitosis producing large number of gametes. It also produces microspores (in anthers) and megaspores (in ovaries).

Initial or Zygotic Meiosis (Haplotonic Pattern): It is found in some algae, fungi and diatoms. Fertilization is immediately followed by mitosis giving rise to cells with the haploid chromosome number. The zygote is the only diploid stage in the life cycle.

You might also read: Mitosis and Its Significance

Steps of Meiosis

The successive meiotic division can be divided into the following two types:

image of Steps of Meiosis

The First Meiotic Division

Meiosis starts after an intermeiotic interphase. In this division, the diploid (2n) number of chromosomes reduces to haploid (n) number chromosomes.

Interphase: Interval between eukaryotic cell division in which growth and synthetic activities take place. The interphase prior meiosis is important because replication of DNA takes place during this stage.

Prophase-I:

It is the most important and longest stage closely similar to the early mitotic prophase. It is divided into the following sub-stages:

  1. Lepotene
  2. Zygotene
  3. Pachytene
  4. Diplotene and
  5. Diakinesis
image of Steps of Prophase-I

Different Stages of Prophase-I

Leptotene: The stage of meiosis in which the chromosomes condense and become visible is known as leptotene or leptonema. The chromosomes become more visible. Hence it is the first step of meiosis. In this stage, the following characteristic features take place:

  • The cells and their nuclei are generally larger than those of the surrounding tissues.
  • The chromosomes are duplicated. In this case, chromosome with two chromatids look single.
  • Under light microscope chromosomes show bead-like thickening called chromomeres, occurring at irregular intervals along their length.
  • Leptomeric chromomeres may show a definite polarization forming loops in which the telomere is attached to the nuclear envelope at a region pointing towards the centrioles. This arrangement is often called a bouquet.
  • In the plant cells, the chromosomes may sometimes form a tangle of threads called the synaptic knot, on one side of the nucleus.
  • In the animal cell, the centrosome divides and goes to opposite poles.

Zygotene: The stage of meiosis in which the homologous chromosomes form pairs with each other is known as a zygote. The important characteristic features of zygotene are:

  • Movement of chromosomes initiates the zygote stage and this movement results from an attractive force that brings homologous chromosomes together.
  • The chromosomes become shorter and thicker.
  • The lengthwise pairing of homologous chromosomes begins.
  • Homologous chromosomes originate one from the male parent (paternal chromosome) and the other from the female parent (maternal chromosome). They are attracted to each other and their pairing takes place. This pairing is known as synapsis. This pairing takes place throughout its length.
  • Pairing takes place not only between homologous chromosomes but also between homologous regions of the chromosomes.
  • During synapsis, a special proteinacious structure called the synaptonemal complex is formed. It is present in the space formed between two homologous chromosomes.
  • The nucleolus is visible.

Pachytene: The stage of meiosis in which two chromatids belonging to different homologues exchange segments of genetic material between them is called pachytene. The important characteristics features of this stage are::

  • The chromosomes are still going shorter and thicker by coiling.
  • Each chromosome is now a bivalent or tetrad (bivalent or tetrad is a pair of homologous consisting of four chromatids. The two chromatids of each homologue are called sister chromatids united by centromere)..
  • The characteristic phenomenon is the exchange of segments; that is the recombination of chromosomal segments between two chromatids belonging to different homologues.
  • Two chromatids belonging to different homologues undergo one or more transverse breaks at the same level. The break is followed by interchange and fusion of broken ends between two homologous chromosomes. This is known as crossing over. Crossing over is the process which involves ensuing redistribution and mutual exchange of hereditary material of two parents between two homologous chromosomes.
  • The nucleolus remains prominent and is found to be associated with the nucleolar organizer region of the chromosome.
image of crossing over

Image Showing Crossing Over

Diplotene: The stage of meiosis in which paired homologues begin to separate is known as diplotene. The characteristic features of diplotene are:

  • The homologous chromosomes repel each other due to the decrease of the force of attraction between them.
  • The separating chromosomes are held together at one or more points in which break and fusion occurred. These points are known as chiasmata. (Chiasmata: The point of chromosomal interchange that becomes visible when the homologues begin to separate at diplotene is known as chiasmata. The number of chiasma(plural) may be one, two or several depending on the length of chromosomes)
image of Chiasma

Chiasma: Image Credit-Wikipedia

Diakinesis: The stage of meiosis during which chromosome contraction increases at the end of this stage the homologues are attached only at the chiasmata. The characteristic features of diakinesis are:

  • The chromosomes have become shorter and thicker as the two bivalents move further away from each other. The homologues remain in contact with each other by their terminal chiasmata. This process is called terminalization.
  • The nucleolus is detached from the chromosome or disappears.

Prometaphase-I:

The characteristic features of prometaphase are:

  • The nuclear envelope disappears.
  • The microtubules get arranged in the form of the spindle in between the two centrioles which occupy the position of two opposite poles of the cell.
  • The chromosomes become greatly coiled in spiral manners and get arranged on the equator of the spindle.

Metaphase-I:

The characteristic features of metaphase-I are:

  • The microtubules of the spindle are attached with the centromeres of the homologous chromosomes of each tetrad.
  • The centromere of each chromosome is directed towards the opposite poles.
  • The repulsive forces between homologous chromosomes increase greatly and the chromosomes become ready to separate.

Anaphase-I:

The characteristic features of anaphase-I are:

  • The homologous chromosomes, each consisting of two chromatids united by a centromere, move towards the opposite poles of the cell.
  • The chromosomes do not separate simultaneously. The short chromosome separates quickly while the separation of long chromosomes is delayed.
  • Unlike mitotic anaphase, in which the chromosomes appear longitudinally single, each chromosome now consists of two distinctly separated chromatids united only at their centromeres.
  • The actual reduction occurs at this stage. The homologous chromosomes that move towards the opposite poles are the chromosomes of either parental or maternal origin.

Telophase-I:

The characteristic features are the following:

  • The endoplasmic reticulum forms the nuclear envelope around the chromosomes.
  • The chromosomes undergo deserialization and become elongated.
  • Nucleoli do not reappear and spindle fibers do not disappear.

Cytokinesis:

In the plant cells: There is a formation of a cell plate between the two groups of chromosomes and thus two daughter cells are formed.

In animal cells: The cell membrane constricts and two daughter cells are formed. The daughter cells thus produced contain a haploid number (n) of chromosomes.

image of Meiosis-I

Meiosis-I: Image credit-Wikimedia Commons

The Second Meiotic Division

It is essentially similar to the mitotic division which divides each haploid meiotic cell into two haploid cells i.e., a number of chromosomes (n) remain the same. The second meiotic division includes the following four stages:

Prophase-II

The characteristic features of prophase-II are:

  • Each centriole divides into two and forms two centrioles.
  • Spindle formation takes place.
  • The nuclear membrane begins to disappear.
  • Each chromosome is made up of two chromatids. The chromatids have widely separated arms.

Metaphase-II

The characteristic features of metaphase-II are:

  • The chromosomes get arranged on the equator of the spindle.
  • The spindle fibers are attached to the centromere of the chromosomes.

Anaphase-II

The characteristic features of Anaphase-II are:

  • The centromere divides into two and thus each chromosome produces two daughter chromosomes called monads.
  • The daughter chromosomes (chromatids) separate out and move towards the opposite poles.

Telophase-II

The characteristic features of Telophase-II are:

  • The chromatids migrate to the opposite poles are now known as a chromosome.
  • The reappearance of the nuclear membrane and nucleolus takes place.
  • Chromosomes disappear due to the hydration of the nucleus.

After the karyokinesis in each haploid meiotic cell, the cytokinesis occurs and thus four haploid cells are produced, These cells may have chromosomes with different genetic combinations due to the crossing over in the prophase-I.

image of Meiosis-II

Meiosis-II: Image credit- Wikimedia commons

Significance of Meiosis

  • Meiosis maintains a definite and constant number of chromosomes in the cell of a particular species.
  • In the animal kingdom. Meiosis leads to the formation of sexual gametes, the eggs (ovum) and the sperm.
  • In the plant kingdom, meiosis occurs at various times during the life cycle (the haploid products may be sexual gametes or asexual spores).
  • Through crossing over, meiosis provides an opportunity for the exchange of the genes and thus it causes the genetic variation among the species.
  • Meiosis is regulated as a compensatory mechanism opposite to syngamy or fertilization.

Difference Between Mitosis and Meiosis

Mitosis

Meiosis

Mitosis occurs in all somatic cells.

Meiosis occurs in germ cells.

Cell divides only once at a time.

Cell divides twice at a time.

Chromosome number remains unchanged after division.

Chromosome number reduced to half after division.

Two haploid cells are formed from a single diploid cell.

Four haploid cells are formed from a singles diploid cell.

Interphase following mitosis is of longer duration.

Interphase following meiosis is comparatively shorter duration.

DNA synthesis takes place in interphase.

DNA synthesis extends up to prophase-I.

Prophase:

The duration of prophase is short.


The duration of prophase is longer.

Prophase is comparatively simple.

Prophase is complicated and is divided into leptotene, zygotene, pachytene, diplotene and diakinasis.

No pairing or synapsis takes place between homologous chromosomes.  

Pairing or synapsis occurs between the homologous chromosomes.

Chromosomes are duplicated at the beginning of prophase.

Chromosomes are duplicated in the late prophase.

No crossing over (chiasma formation) takes place.

Crossing over takes place.

Metaphase:

The chromatid occurs in the form of dyads. 


The chromatids of the two homologous chromosomes occur as the tetrads.

The centromeres of the chromosomes remain directed towards the equator and arms of the chromosomes remain directed towards the poles.

The centromeres of the chromosomes remain directed towards the poles and the chromosomal arms remain directed towards the equator plane.

Anaphase:

Division of centomeres takes place during anaphase.


There is no centromeric division during anaphase-I but take place during anaphase-II.

The chromosomes separate simultaneously during an anaphase.

Short chromosomes separate early early separation of long chromosomes delayed.

Telophase:

Spindle fibers disappear completely in telophase,


Spindle fibers disappear completely during telophase-I.

Nucleoli reappear. 

Nucleoli do not reappear in telophase-I.

Cytokinesis occurs in Telophase.

Cytokinesis occurs in Telophase I and Telophase II.

Steps of Mitosis: Prophase, Prometaphase, Metaphase, Anaphase, Telophase.

Steps of Meiosis: Prophase I, Metaphase I, Anaphase I, Telophase I,
Prophase II, Metaphase II, Anaphase II, Telophase II

Significances:

The chromosomes number in each daughter cell remains the same like the parent cell,

In meiotic cell division, the chromosome number is reduced to half in the daughter cells.

The genetic constitution of the daughter cell identical to that of parent cells.

The genetic constitution of the daughter cells differs from that of the parent cell. The chromosome of the daughter cells usually contains a mixture of maternal and parental genes. 

Concluding Remarks

Meiosis is the cell division process by which sexually reproducing organisms can produce their gametes. It includes two divisions such as Meiosis-I and Meiosis-II and produces four haploid cells.  All animals and plants produce their future generations through the process of meiosis. In this process, the parent cell divides two times and produces four daughter cells in which each daughter cell contains half the original amount of genetic information.

Mitosis: Definition, Stages of Mitosis and its Significance

The old cells divide to produce new cells. In this case, a single old cell divides into two cells and these two new cells again divide to produce four cells and so on. Generally, this process is known as cell division where a parent cell divides into two or more daughter cells.

The capability of cell division in living organisms is unique and produces more and more cells. In this way, almost two trillion cells are produced in human body every day. According to biologist, the number of cells in human body is around 37 trillion. During the cell division, the parent cells divide to produce two daughter cells and this process occur cyclically, known as the cell cycle. In the perspective of the cell cycle, mitosis is the one kind of the division process where the DNA of the cell's nucleus breaks down into two equal sets of chromosomes.

Mitosis replaces old, worn-out cells with new ones throughout an organism’s life. Generally, the goal of mitosis is to make sure that each daughter cell gets an absolute, full set of chromosomes. If the cell contains too few or too many chromosomes, they usually don’t function perfectly. Because, these cells cannot survive at all; or they can cause cancer into your body which leads to death.

Based on the type of cells, the cell division occurs two ways: Mitosis and meiosis. Mitosis is the process of one type of cell division by which the mother cell is precisely divided into two new daughter cells that have two new chromosome sets and each daughter cell is genetically identical to the original mother cell while in meiosis, a single cell divide into four daughter cells where the number of chromosomes is reduced by half to produce haploid gametes.  In this case, mitosis cell division is good for basic growth, maintenance and repair. But in meiosis, the number of chromosomes is reduced by half which provide for genetic diversity that is important for sexual reproduction.

The Cell Cycle or Mitotic (M) Cycle

The mitosis is a cell division process which successfully make the new diploid cells. In continuously dividing cells, an individual cell passes through two main phases of cell or the mitotic cycle or the cell cycle. A growing cell undergoes a cell cycle that consists essentially of two phases i.e., interphase and mitotic phase. The interphase is considered as resting phase of the cell whereas mitotic phase is the most important part of the cycle in which cell divides.

image of Cell cycle

Cell Cycle: Image credit-Wikimedia Commons

Interphase

The interphase is the interval between cell divisions in which growth and synthetic activities take place. This phase is also known as inter-mitotic phase. In this phase, a cell gets ready, grows by gathering nutrients, and energy.  The cell also increases in size, produces organelles and DNA is doubled by making a copy during this period. It is divided into the following three phases:

S-phase or Synthetic period

This phase is specific part of interphase in which DNA synthesis occurs.  S-phase is replaced and followed by two gap periods of interphase G2 and G1. In this phase, the cell makes a complete copy of the DNA in its nucleus. It also copies a microtubule-organizing structure called the centrosome which is essential to separate DNA during M phase.

G2-period

It is the interval between the end of S-phase and the start of mitosis. In this phase, the cell grows more, builds proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G2 period ends and mitosis begins.

G1 period

It is the interval between the end of mitosis and the start of S-phase. It is also called the first gap phase where the cell grows physically larger, copies organelles, and makes the molecular building blocks. 

During interphase, the following some characteristic features are observed:

  • Growth of both nucleus and cytoplasm occur to maintain normal size of the cell.
  • The nucleo-membrane remains intact.
  • The chromosomes remain diffused in the nucleoplasm as long coiled thread like chromatin fibres.
  • The amount of DNA becomes two times more than in the original diploid cell.
  • The nucleolus becomes distinct.
  • In animal cell, two centrioles and the centrospheres around the centriole become distinct.
  • The most significant events are exact replication of the DNA in the nucleus and related to this, the duplication of the chromosomes from one chromatid to two identical chromatids.

Duration of Cell Cycle

The duration of cell cycle varies greatly from one type of cell to another.  For a mammalian, cell growth in culture with a generation time of 16 hours. The length of different period would be G1=5 hours, S=7 hours, G2= 3 hours and mitosis 1 hour. The mitotic periods are relatively constant in the cells of same organism. The G1 period is the most variable in length. Depending on the physiological conditions of the cells, it may last for days, months or years.

Phases of Mitosis

Mitosis consists of the following four basic phases:

  • Prophase
  • Metaphase
  • Anaphase and
  • Telophase

In some textbooks, they list five phases of mitosis such as prophase, prometaphase, late metaphase, anaphase, and telophase. In this case, they divide the metaphase stage into an early phase, known as prometaphase and a late phase, known as late metaphase.

In the mitotic (M) phase, the cell splits it’s DNA into two sets and divides its cytoplasm, for the formation of two new cells. The main mitotic cell division occurs during mitotic phase.  The series of events that make up the cell cycle or mitotic cycle that begins at the end of the G2 period of interphase and terminates at the beginning of G1 period of a new interphase. The main stages of mitosis are mainly divided into Karyokinesis and cytokinesis.

Karyokinesis: It is the division of nuclear material which starts with doubling of chromosome in parent nucleus followed by distribution of between two daughter nuclei in equal proportion.  Karyokinesis includes prophase, metaphase, anaphase and telophase.

Cytokinesis: The cytokinesis can take place simultaneously with anaphase and telophase or it can occur at the later stage.

Prophase

It is the first stage of mitosis in which the chromosomes shorten and become visible within the nucleus followed by the dissolution of the nuclear envelope.  In this stage, most dramatic changes occur both in the nucleus and cytoplasm.

Changes in the Nucleus

  1. The chromosomes shorten, thicken, become stainable and form X-shaped structures that can be seen under a microscope. Shortening is due to loss of water from chromatin fiber and spiralization of threads takes place.
  2. Each chromosome consists of two longitudinal subunits called the chromatids, containing identical genetic information. They are in very close association with each other all along their length. These chromatids are coiled around each other.
  3. During early prophase, the chromosomes are evenly distributed in the nucleoplasm but as prophase progresses the chromosomes migrated towards the nuclear membrane.
  4. The nucleoli gradually diminish in size and eventually the nucleolar material becomes dispersed.

Changes in the Cytoplasm

  1. The centriole (in animal cells) which had undergone duplication during interphase now begin to move towards opposite poles.
  2. The movement of centriols are due to their being pushed apart by growth of the spindle fibres between them. In animal cells or in the lower plants, fibrils appear like spokes of wheel around each centriole to form asters. The ester, the centrioles and the spindle together make up the structure called mitotic apparatus or achromatic figure.

Metaphase

It is the second stage of mitosis in which the chromosomes are attached to spindle fibres in the equatorial plane of the cell. The metaphase can be divided into two phases namely: early metaphase or prometaphase and late metaphase.

Early Metaphase or Prometaphase

The transition between prophase and metaphase sometimes called prometaphase. The following changes occur in this phase:

  1. It is the short period in which the nuclear membrane disintegrates.
  2. The chromosomes begin to proceed towards the equator.

Metaphase

The following changes occur in this phase:

  1. The chromosomes have reached the central or equatorial position of the spindle. They are lined up in one place and thus form the equatorial plate or metaphasic plate.
  2. The centrioles lie on the equator of the spindle.
  3. The centromeres lie in a plane equidistant from the spindle pole and the arms are directed towards the poles.
  4. It is the stage at which chromosomes have reached the point of maximum contraction.
  5. The mitotic spindle fibres attach to each of the sister chromatids.

Anaphase

  1. Actually, it is the 3rd stage of mitosis in which the centromeres and daughter chromosomes separate and begin to move toward opposite poles. The following changes occur in this phase:
  2. The centromeres of the chromosomes divide and the two chromatids of each pair separate. They are called sister chromatids.
  3. The sister chromatids migrate towards the poles due to shortening of spindle fibres attached to the centromeres. In this case, the sister chromatids are pulled apart by the mitotic spindle which pulls one chromatid to one pole and the other chromatid to the opposite pole.
  4. Pulling causes the shape of chromosomes. In this case, chromosomes look like v (metacentric), J (acrocentric) or L (submetacentric) shape.

Telophase

It is the 4th or last stage of mitosis in which the chromosomes uncoil and become surrounded by new nuclear envelopes. This stage begins at the end of the polar migration of the daughter chromosomes. The following changes occur in this phase:

  1. The chromosomes uncoil.
  2. A new nuclear membrane is formed around each mass of chromatin to create two nuclei.
  3. Nucleoli reappear at nucleolar organizer present in one pair of chromosomes.

Cytokinesis

During cytokinesis, the cytoplasm of a single eukaryotic cell separates into two daughter cells. In cytokinesis process, the single cell pinches in the middle and the spindle apparatus partitions and forms two separate daughter cells with a full set of chromosomes within a nucleus. This process also ensures that chromosome number and complement are maintained from one generation to the next generation identical to those of the mother cell. After finishing cytokinesis, each daughter cell enters the interphase and repeats the cell cycle.

image of Mitosis
image of Mkitosis

Stages of Mitosis:Image credit-Wikimedia commons

In Animal Cells

A cleavage furrow appears at the outer edges of the cell and midway between the poles at the beginning. This furrow of constriction becomes progressively deeper as the spindle breaks down. Finally, the ingrowing constriction joins and cleaves the cell in two daughter cells.

image of Cytokiness in-animal cell

Cytokinesis in animal cell

In Plant Cells

Cytokinesis in plant cell involves the formation of a cell wall between the daughter nuclei. This begins as a cell plates or phragmoplast formed by aggregation of vesicles from the Golgibodies. These vesicles fuse with each other to form cell membranes and cell walls, thereby divide into two parts. The spindle, is then disintegrates and cell division is completed. Thus two cells, each with a nucleus containing a full complement of chromosome have been formed from the original cell.

image of Cytokiness in plant cell

Cytokinesis in plant cell

Plant Vs Animal Cytokinasis

Plant Cytokinesis

Animal Cytokinesis

It is the cytoplasmic division where the plant cell separates into two daughter cells.

It is also the cytoplasmic division where the animal cell separates into two daughter cells.

It occurs by forming of a cell plate at the middle of the cell due to have a rigid cell wall.

It occurs by the formation of a cleavage furrow.

In this case, cell walls are formed.

In this case, cell walls are not formed.

Cell membrane does not constrict.

In this case, cell membrane constricts.

At the equatorial plane, the cell plate is formed from the vesicles with cell wall materials released from the Golgi apparatus.

At the equatorial plane, non-muscle myosin II and actin filament assembles and contractile ring forms in the middle of the cell at the cell cortex.

Significance of Mitosis

The importance of the mitosis has been summarized as follows:

  • Mitosis helps the growth and development of organs and the body of the organisms.
  • It helps to maintain the proper size and shape of the cell.
  • Mitotic cell division replaces the old decaying and dead cells by forming the new cells.
  • Unicellular organisms mainly reproduce and multiply by mitosis.
  • The essential feature of mitosis is that the chromosomes are distributed equally among the two daughter cells.
  • In mitosis, a mechanism for the duplication of chromosomes and for linear arrangement of genes takes place.
  • Mitosis helps to maintain the balance between DNA and RNA contents with the nuclear and cytoplasmic contents of the cells.
  • Through the mitosis, equilibrium is maintained in the amount of DNA and RNA contents.
  • Mitosis also helps the organisms in the asexual reproduction.

RNA: Definition, Types, Structure and Functions

Johannes Friedrich Miescher (a Swiss physician and biologist) first discovered the Nucleic acids as ‘nuclein’ from the nucleus in 1869. Severo Ochoa de Albornoz (a Spanish-American physician and biochemist) discovered the RNA synthesis mechanism in 1959. He won the Nobel Prize jointly with Arthur Kornberg in Physiology or Medicine for his discoveries. In 1965, Robert W. Holley sequences 77 nucleotides of yeast tRNA.

Ribonucleic acid or RNA is an essential biological macromolecule. Generally, it helps to exchange the hereditary information encoded by DNA into proteins. The nucleic acid of living cell having ribose sugar in its nucleotides and perform multiple vital roles in the coding, decoding, regulation and expression of genes is called Ribonucleic acid or RNA.

In prokaryotic cell these are found in cytoplasm, chromosome, ribosome, nucleolus, plastid and mitochondria. In eukaryotic cells, 90% RNA present in cytoplasm and 10% in other structures. In some virus, RNA exists as genetic material.

Physical Structure of RNA

Primarily RNA is single-stranded particle with an intra-strand pairing yet in their secondary structure; there are a few U shaped loops. It can show a broad twofold helical structure and can also form different tertiary structures.

image of RNA-Physical structure

Chemical Structure of RNA

RNA molecule is a polymer of ribonucleotide. Each ribonucleotide consists of the following molecules:

  • One molecule of ribose pentose sugar,
  • One molecule of inorganic phosphoric acid and
  • One molecule of nitrogenous base.

Each nucleotide in a RNA molecule has one of four nitrogenous bases: adenine, guanine, cytosine and uracil. The first two are purine and the later two are pyrimidine bases.

Types of RNA

RNA is of two main types, such as:

1. Genetic RNA or gRNA: When RNA functions as genetic materials then it is known as genetic RNA, e.g. RNA of some viruses.

2. Non-genetic RNA: When RNA takes part in only protein synthesis, then it is called non-genetic RNA, e.g. RNA of eukaryotic and prokaryotic cells.

Non-genetic RNA is further divided into the following three types:

  1. Ribosomal RNA or rRNA
  2. Messenger RNA or mRNA
  3. Transfer RNA or tRNA

Ribosomal RNA or rRNA: It makes up about 80% of the total RNA in a cell. These are synthesized in nucleolus and occur in ribosome, the protein factories of the cells. Ribosomal RNA is composed of unbranhed, flexible polynucleotide chain. This chain remains coil in low ionic concentration but its nitrogen bases form helical part in high ionic concentration. In such case, adenine bound with uracil and guanine bound with cytosine.

Eukaryotic rRNA is of four types: 28S rRNA,  18S rRNA,  5.8S rRNA,  and 5S rRNA.

image of Ribosomal-RNA

Image showing Ribosomal RNA (rRNA)

Functions of rRNA

  • Ribosomal RNA gives a procedure for decoding mRNA into amino acids and interrelates with tRNAs during translation.
  • It comprises the predominant material within the ribosome. During protein synthesis.
  • It guarantees the proper alignment of tRNA, mRNA, and ribosome.
  • It catalyzes during peptide bond formation between amino acids.

Messenger RNA or mRNA

French scientists François Jacob and Jacques Monod coined the name mRNA in 1961. mRNA is a single-strand made of up to several thousand nucleotides. It is created as complementary strand of DNA hence it has base sequences as like as in DNA. In its linear structure, mRNA has two non-coding ends and middle coding zone. The two ends of mRNA recognized as 5´ leader and 3´ trailer end. mRNA makes up 3-5% of the total RNA in a cell.

mRNA is a copy of the hereditary information produced by transcription from the cell’s original blueprint, DNA. This hereditary information is brought to the protein factories of the cells, ribosome for using as instruction for the formation of proteins.

image of mRNA

Image showing mRNA

Functions of mRNA

  • mRNA is transcribed from the DNA template in the nucleus and carries coding information to the sites of protein synthesis in the ribosomes.

Transfer RNA or tRNA

Transfer RNA or tRNA is a relatively small, clover leaf form of RNA that transfers a particular amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis during translation. In this case, each of the 20 amino acids which have a specific tRNA that binds with it to form proteins. The tRNA is made up of 70 to 95 nucleotides. It is the essential component of translation and it performs to transfer of amino acids during protein synthesis as a main functions. Hence, it is called transfer RNA or tRNA.  tRNA is also called adaptor molecules because it acts as adaptor in the transformation of the genetic sequence of mRNA into proteins. Sometimes tRNA ia also called soluble, or activator RNA.

Robert Willium Holley et al. (1965) proposed the clover leaf model structure of tRNA. He awarded the Nobel Prize in Physiology or Medicine in 1968 with Har Gobind Khorana and Marshall Warren Nirenberg for describing this model. According to this model the single polynucleotide chain of tRNA is folded upon itself to form five arms. The arms are:

  1. Acceptor arm
  2. Dihydrouridine (DHU) arm or D arm
  3. Anticodon arm
  4. Variable arm and
  5. Thiamine psedocytosine or TΨC arm.

tRNA also have DHU loop, variable loop, anticodon loop, T-loop or TΨC loop and amino acid acceptor end. It has four normal bases A, G, U, C and some unknown bases like isonine (I), dihydouridine, psedouradine, etc. Both end of single chain of tRNA (5´-3´) exist aside.

image of Clover leaf model of tRNA

Image showing Clover Leaf Model Structure of tRNA

Functions of tRNA

  1. It identifies and transports the correct amino acid molecules to the site of protein synthesis in the ribosome.
  2. It primarily is familiar for carrying amino acids.
  3. It also takes part in the process of building proteins.

RNAa can also be broadly divided into the following types:

  1. Coding (cRNA) and
  2. Non-coding RNA (ncRNA)

Non-coding RNAs (ncRNA) are of the following two types, such as:

  1. Housekeeping ncRNAs (tRNA and rRNA) and
  2. Regulatory ncRNAs.

Non-coding RNA (ncRNA)  is further divided into the following two types based on their size such as:

  1. Long ncRNAs (lncRNA): It consists of at least 200 nucleotides, and 
  2. Small ncRNAs: It consists of fewer than 200 nucleotides.

Small ncRNAs are subdivided into the following five types:

  1. micro RNA (miRNA),
  2. Small nucleolar RNA (snoRNA),
  3. Small nuclear RNA (snRNA),
  4. Small-interfering RNA (siRNA), and
  5. PIWI-interacting RNA (piRNA).

​The miRNAs are about 22 nucleotides long and have particular importance. In most eukaryotic cells, they perform to function in gene regulation. They also obstruct gene expression by binding to target mRNA. Some miRNAs can regulate target genes which lead to tumour progression and tumorigenesis.  piRNAs are about 26 to 31 nucleotides long which are found in most animals. They can regulate the expression of jumping genes or transposon that move from one location to another on a chromosome. These genes were first identified by Barbara McClintock.

Concluding Remarks

Definitively, the RNA is as significant as DNA into molecular investigations as the evaluation of the gene expression is relies upon the complete mRNA present into the specific tissue. mRNA is fundamental to the procedure of transcription, while tRNA is essential to the procedure of translation, and rRNA makes up the ribosomes in which translation occurs. The amount of expression of gene can be estimated by utilizing the Reverse transcription polymerase chain reaction  technique or RT-PCR technique through the evaluation of RNA.

Chromosome: Types, Structure and Functions

The word chromosome is derived from the Greek ‘Chroma’ meaning color and Greek ‘soma’ meaning body. The chromosome is the gene bearing rod-shaped structure which became clearly visible during the cell division and typically present in the nucleus that carries hereditary information in the form of genes.

In 1942, Karl Nagli observed the rod-like chromosome in the nucleus of plant cells and it was probably the first to report their sighting. The German scientists Schleiden first recognized the structures of chromosomes. W. Hofmeister drew the figure of the chromosome of pollen mother cells of Tradescantia. Walter Fleming (1979) introduced the term chromatin, the thread-like material of the nucleus and in 1882; he gave the accurate account of chromosome (nuclear filament). W.S. Sutton and T. Boveri (1902) suggested that the chromosomes are the bearer of heredity. DuPraw proposed the Folded Fibre Model of chromosome in 1965.

Chromosome Numbers

In a particular species, all individuals have same number of chromosomes. Chromosomes number is of great importance in the determination of phylogeny and taxonomy of the species.

Gametes (sperm and ovum) contain only one set of chromosomes. This number is called haploid number (n). The somatic or body cells usually contain two sets of chromosomes. This number is known as diploid number (2n). The haploid number of chromosome of male (n) and female (n) gametes combined and form 2n, i.e., diploid number of chromosomes. Sometime, the somatic cells of some animal as well as plant contain more than two sets of chromosomes. They are called triploids (contain three sets, 3n) and tetraploid (contain four sets, 4n), etc. The lowest haploid chromosomes number recorded in eukaryotes is two (n=2); e.g., Mesotoma ucamara (flatworm). In Ascaris megalocephalaus univalens only one pair of chromosomes are found. Belar (1926) has been observed that Aulakantha (a primitive animal) contains maximum number of chromosomes, i.e., 1600 (2n).

Chromosome number (2n) in some common plants and animals

Plant Species

Chromosome number

Animal Species

Chromosome number

Rye

14

Common fruit fly

8

Einkorn wheat

14

House fly

12

Maize

20

Mosquito

6

Durum wheat

28

Goldfish

100-104

Bread wheat

42

Toad

22

Cultivated tobacco

48

Frog

26

Yeast

4

Turtle 

32

Onion

16

Pigeon

80

Cabbage

18

Cow

60

Radish

18

Dog

78

Papaya

18

Domestic cat

38

Rice

24

Domestic sheep

54

Tomato

24

Donkey

52

Tea

30

Monkey

42

Sunflower

34

Elephant

56

Pea nut

40

Chimpanzee

48

Coffee

44

Guinea pig

64

Potato

48

Guppy fish

46

Cotton

52

Horse

64

Sugarcane

80

Human

46

Size and Shape of the Chromosome

The size of the chromosome varies from species to species. The length of the chromosome remains constant for a particular species and it may vary from 0.2-2 µm. The length of human chromosome is 6 µm.

The shape of the chromosome is changeable from phase to phase in the continuous process of cell growth and cell division. A chromosome contains a clear zone called Centromere or Kinetochore.

Types of Chromosomes

According to the position of centromere, the chromosome may be of the following types:

Telocentric: It is ‘I’ shaped or rod-like chromosome during anaphase stage of cell division. In this case, the centromere is situated at the terminal end of the chromosome. One arm is very long and the other is absent. This type of chromosome is very rare.  Human  does not possess telocentric chromosome but the standard house mouse karyotype possesses only one telocentric chromosomes.

Acrocentric: It is J or rod-shaped chromosome in which the centromere is situated near the end of the chromosome. One arm is very long and the other is very small. Five acrocentric chromosomes such as  13, 14, 15, 21, 22 are present in the human genome.  The Y chromosome is also acrocentric.

Submetacentric: It is ‘L shaped chromosome in which the centromere is located slightly away from the mid-point of the chromosome. It contains a slight shorter arm than the other chromosome. In this case, the arms are unequal in length. In human submetacentric chromosomes are 4 through 12. 

Metacentric: It is also ‘v’ shaped chromosome in which the centromere is situated in the middle of the chromosome. The two arms of the chromosome are nearly equal in length. Five chromosomes are considered metacentric chromosome in a normal human karyotype (1, 3, 16, 19, and 20).

image of Chromosomem types

Based on the number of cetromeres, the chromosomes may be of the following types:

Monocentric: The chromosomes of most organisms contain only one centromere. This type is called monocentric chromosome.

Dicentric: When chromosome contains two centromeres then such chromosome is known as dicentric chromosome. In this case, one centromere is present in each chromatid.

Polycentric: When chromosome contains more than two centromeres then such chromosome is known as polycentric chromosome.

Acentric: Sometime a chromosome may undergo a break into two, so that only one part has the centromere while the other is without centromere. The part lacking the centromere is called acentric chromosome.

Diffused or non-located: In this case, centromere is indistinct throughout the whole length of chromosome.

Based on functions, chromosomes are of the following types:

Functionally, the eukaryotic chromosomes are classified into two types: autosomes and sex-chromosomes.  

Autosomes

Most of the chromosomes in the cell are autosomes which are responsible for the determination of the body structure and functions.

Sex-chromosomes

There are one or usually two sex chromosomes which are responsible for determination of sex organ structure and their functions. The sex chromosomes are also known as accessory chromosomes, allosomes or heterochromosomes. They carry genes for determination of sex. The sex chromosomes are of two kinds, X chromosomes and Y chromosomes. These two chromosomes such as X and Y differ in size and morphology. One of the sex chromosomes has a pair of identical sex chromosomes (XX), the other may have a single sex chromosome which may be (XO) or paired with Y chromosomes (XY).

Human gametes are not identical with respect to the sex chromosomes. The male is heterogametic (XY) and produce either X or Y bearing spermatozoa in equal proportion. The female is homogametic (XX) produces only X bearing ovum. In human species (Homo sapiens), the total number of chromosome in somatic cell is 46 (23 pairs). Out of the 46 total chromosomes of man 22 pairs are autosmoes and one pair is sex chromosomes.

Structure of Chromosome

During metaphase of the cell division, the chromosomes are clearly visible under light microscope. Electron microscope studies revealed that the chromosomes are made up of the following parts:

  1. Pellicle
  2. Matrix
  3. Chromatid
  4. Chromomere
  5. Centromere
  6. Primary constriction
  7. Secondary constriction
  8. Satellite and
  9. Telomere, etc.

Pellicle: The thin membranous structure which remains outside  the chromosome is known as pellicle.

Matrix: The semi-liquid jelly like substance present within the pellicle is known as matrix. It is supposed that during prophase, it is formed from the nucleolus and during telophase matrix reproduces nucleolus. The concept of presence of pellicle and matrix in the chromosome has been rejected by recent electron microscopic observation by Robertis (1970).

Chromatid: It is two symmetrical twisted thread-like structures containing a single DNA molecule and joined together at primary constriction. Each chromosome consists of two longitudinal subunits, the chromatids, which are in very close association with each other all along their length. This sister chromatids consists of a number of longitudinal subdivisions known as chromonemata (singular: Chromonema). According to species, the chromonema may be composed of two, four or more microfibrils. Microfibrils contain genes. The microfibrils of the chromonema remain coiled with each other which are as follows:

  1. Paranemic coils: The microfibrils which are loosely coiled so that they can be easily separated from each other are called paranemic coils.
  2. Plectonemic coils: The microfibrils remain inter-twisted so intimately that they cannot be separated easily are known as plectonemic coils.
image of Paranemic and plectonemic chromosome

Centromere: The point of attachment of sister chromatids and side of chromosome attachment to the mitotic spindle fibre is known as centromere.

Most of the chromosomes possess usually two constrictions called primary and secondary constriction, respectively. The primary constriction is called centromere. The centromere is the structure concerned with movement of the chromosome. Without it, a chromosome cannot orient on the spindle and the chromatids cannot segregate from each other.

In eukaryotic cell, centromere plays an important role for segregating proper chromosome during mitosis and meiosis. It’s essential roles include sister chromatid adhesion and separation, chromosome movement, microtubule attachment, mitotic checkpoint control and establishment of heterochromatin. 

Chromomere: The bead-like structures of the chromonema are known as chromomere.  It is also known as idiomere. The chromonma of thin chromosomes of mitotic and meiotic prophase have been found to contain alternating thick and thin parts. Thus it gives an appearance of a necklace in which several beads (thick parts) are located on a string (thin parts). The beads are known as chromomeres and the string (fiber) like structure in between the chromomeres is termed as the interchromomeres.

image of Kinetochore

Kinetochore

  • Kinetochore: The proteinaceous disc attached to the centromeric chromomares and with which spindle microtubules are attached is called kinetochore. During prometaphase and telophase, kinetochore is made up of three zones. Microtubules found embedded in all the three layers.
  • Centromeric Chromomeres: During the metaphase stage of the cell division, the chromomere contains two chromatids. At this time, four granules are seen within the centromere.  These granules are known as centromeric chromomeres. The granules are about 0.5 µm in size which are arranged in square.

Primary Constriction: Comparatively a lightly stained constricted region of the chromosome having its centromere which is located at a particular position is known as primary constriction. It divides the chromosomes into arms.

Secondary constriction: In addition to the primary constriction,n the arms of the chromosome may show one or more constrictions which are called secondary constriction. It may present in one or both the arms. One of the secondary constrictions is known as nucleolar organizer (secondary constriction I).

Certain secondary constriction that contains the genes for ribosomal RNAs and attaches with the nucleolus is known as nucleolar organizer. In human, nucleolar organizers are located in acrocentric chromosomes (no. 13,14,15,21, and 22). These organizers separate the small rounded piece of chromosome called satellite from the rest of the chromosome.

Satellite: The round or elongated or knob-like part present beyond the nuclear organizer of the chromosome is known as satellite. The satellite remains connected with the rest of the chromosome by a thin chromatin filament. Satellites bearing chromosomes are called SAT-chromosomes.

Tellomere: The tip of the chromosome contains the ends of long DNA a molecule that makes up each chromatid is known as telomere. It is specially modified ends of the chromosome which is used for attachment to the nuclear envelope. The ends of the chromosome are associated with the nuclear envelope from telophase to prophase.

image of Chromosome structure

Chemical Structure of Chromosome

Chemically, chromosomes are mainly composed of nucleic acids and protein. There are trace amount of lipid, enzymes, Ca2+, Mg2+ are exist in the chromosome.

Nucleic Acid: The largest molecule of the cell is the nucleic acid. Chromosomes have two types of nucleic acids: DNA (Deoxyribo Nucleic Acid) and RNA (Ribo Nucleic Acid).

  • DNA (Deoxyribo Nucleic Acid): It is the permanent component of chromosome. Among all components DNA contribute 45%. Actually this is a double-strand polymer of nucleotide molecules. Each nucleotide again consists of a nitrogen bases (adenine, guanine, cytosine and thymine), a deoxyribose pentose sugar and an inorganic phosphate molecule.
  • RNA (Ribo Nucleic Acid): It is temporary component of eukaryotic chromosome but permanent genetic component of some virus. Among all components RNA contribute 0.2-1.4%. Actually it is a single-strand polymer of nucleotide molecules. Each nucleotide again consists of a nitrogen bases (adenine, guanine, cytosine and uracil), a ribose pentose sugar and an inorganic phosphate molecule.

Protein: The main structure of chromosome is made by protein. Nucleic acid remains linear arranged within protein. Two types of proteins are found in chromosome.

  • Alkaline Proteins: This type of protein bears low ionic properties. Most of chromosomal alkaline proteins are of histone protein. The amount of histone protein is equal to the DNA i.e., the ratio of histone protein and DNA is 1:1 in chromosome. Another type of alkaline protein protanmine is found only in sperm.
  • Acidic Proteins: These are non-histone proteins having high ionic properties. Chromosomes have several types of acidic proteins but they content more DNA polymers and RNA polymers.

Functions of Chromosomes

Chromosomes carry all necessary information to carry out different functions of the cell or incense it is the genes in the chromosomes which guide the cell in performing different functions like:

  1. Guiding the cell in the cell division;
  2. It plays an important role in inheritance of characters from generation to generation;
  3. Guiding protein synthesis;
  4. It controls all the metabolic functions of the cell;
  5. Self repairing;
  6. It plays roles in sex determination;

Roles of Chromosome in Cell Division

Chromosomes are essential for the process of cell division and responsible for the replication, division and creation of daughter cells that contain correct sequences of DNA and proteins. Proteins make up for one of the most important components of the human body.

For building muscles and tissues of the body, chromosomes play important role. In this case, the body produces thousands of enzymes, like DNA replication enzymes with the help of chromosomes which are essential for growth and repairs.  Chromosome bears genes which are responsible for protein synthesis.

During cell division stages, the chromosome is responsible for the replication and distribution of DNA amongst new cells. Cell division must occur for an organism to function properly for maintaining growth, repair, or reproduction.

Reproductive cells (ovum and sperms) contain the correct number of chromosomes for building correct structure otherwise; resulting progeny may fail to develop properly. In humans, defective chromosomes made up of joined pieces of broken chromosomes cause one type of leukemia and some other cancers. During cell division, chromosomes ensure DNA is accurately copied and distributed in the vast majority of the cell division.

Chromatin and Its Types

Chromatin is the thread-like coiled elongated structure present in the nucleoplasm. It can be stained with basic stain. Emil Heitz  discovered the term Heterochromatin and Euchromatin in 1928. Chromatin is mainly of two types:

  1. Heterochromatin and
  2. Euchromatin

Heterochromatin: During interphase, the regions of the chromatin of the  condensed chromosome which remain tight folding and coiling and which stains deeply is known as heterochromatin. It is present in the (a) telomere (b) centromere and its both sides, (c) satellite and (d) both sides of the secondary constriction. There are two types of heterochromatins recognized (i) constitutive heterochromatin which is permanently condensed in all types of cells and (ii) facultative heterochromatin which is condensed only in certain cell types or at a special stage of development.

Euchromatin: During interphase, the portion of the chromatin which remains less tightly and which stains less deeply is known as euchromatin. The bulk of chromosome is made up of euchromatin.

Chemical composition of Eukaryotic chromatin

The chromatin of eukaryotic cell consists of four types of molecules:

  1. DNA,
  2. RNA,
  3. Histone (basic protein), and
  4. Non-histone protein.

DNA and histone combine to form nucleoprotein, a conjugated protein. They remain with the ratio 1:1. The RNA ad acid protein varies in amount quite widely from one kind of cell to another. Primarily chromosome contains 90% DNA + basic protein and 10% RNA+ acid protein. Histone is the basic protein rich in the amino acids, lysine and arginine. The chemistry of chromatin of prokaryotic cell is made up of only naked DNA.

Difference between Heterochromatin and Euchromatin

Heterochromatin

Euchromatin

It is composed of 250 Å fibres.

It is composed of mostly 30-80 Å  fibres.

In heterochromatin region, the chromatin fibres are more tightly folded.

In euchromatin region, the chromatin fibres are loosely coiled.

t is seen in eukaryotic cells only. In this case, it is located at the periphery of the nucleus.

It is seen in both eukaryotic and prokaryotic cells. In this case, it is located in the inner body of the nucleus.

It stains darkly.

It stains lightly.

It contains more DNA.

It contains less DNA.

It is genetically immobile.

It is genetically active.

It does not synthesize messenger RNA(m-RNA) and protein.

It synthesizes DNA and messenger RNA during interphase.

The cross-over frequency is less.

The cross-over frequency is more.

It does not participate in transcriptional activity.

It participates in the transcriptional activity.

It shows heteropycnosis.

It does not show heteropycnosis.

It can control the structural integrity of the cell. It also regulates the gene expression.

It creates genetic variations and enhances the genetic transcription.

In this case, phenotype is not affected due to addition or loss of heterochromatin region.

In this case, phenotype is affected due to addition or loss of euchromatin region.

It is more affected by temperature, sex, age, etc.

It is less affected by temperature, sex, age, etc.

Giant Chromosome

At certain particular stages, some cells contain large nuclei with giant or large sized chromosomes. They are generally of two types:

  1. Polytene Chromosome and
  2. Lambrush Chromosome

Polyten Chromosome

It is also known as salivary gland chromosome. This type of chromosome first observed by Balbiani (1881) in the salivary glands of midge Chironomous and hence it is called salivary gland chromosome. Such chromosomes were observed in several other insects like Drosophila, mosquitoes, etc. As this chromosome contains many chromonema, so Koller (1888) suggested the name as polytene chromosome (poly, many).

The salivary gland chromosomes of Drosophila do not coil and the four pairs of chromosomes have total length of about 2000 µm whereas the somatic chromosome is only 7.5 µm in length. These giant polytene chromosomes display particular banding patterns. A series of dark bands alternate with clear zones called interbands. The dark bands represent regions where the DNA is more tightly coiled while in the interbands the DNA fibers are folded more loosely. There are 5000 bands in four Drosophila chromosomes. One of the most remarkable characteristics of polytene chromosome is that it is possible to visualize in them the genetic activity of specific chromosomal sites at local enlargements called puffs. The chromonema of the polytene chromosomes gives out many series of the loops laterally. These rings are known as Balbiani rings which are rich in DNA and m-RNA.

image of Polytene Chromosome

Lampbrush Chromosome

These chromosomes with highly extended lateral loops of DNA are found in animal of yolk rich oocytes (shark, amphibians, reptiles and birds) at the diplotene (diplonema) stage of meiosis. It is also found in the nucleus of Acetabularia and even in plants. The giant size of this chromosome is due to increase in size of cromonema. Lampbrish chromosomes are so named because of their many lateral loops of DNA which give them a characteristic appearance reminiscent of bottle washing brushes. It is 3 times longer (5900 µm) than polytene chromosomes. The axis of lampbrush chromosome has a row of chromomeres. Each chromomere has two lateral loops which are rich in RNA. Each loop in turn has an axis formed by single DNA molecule.

image of Lampbrush Chromosome

Lampbrush Chromosome

Cilia and Flagella: Structure and Functions

The hair-like extended portion of the cell surface bounded by the cell membrane and containing microtubules and responsible for cell motility are known as cilia and flagella. The cilia and flagella are widely distributed in both the animal and plant cells.

Flagella (singular = flagellum) are complex filamentous, long, thread-like structures that extend from the plasma membrane. They are un-branched, mostly composed of the protein flagellin. The cell contains one or a few flagella, which are primarily used for locomotion. They protrude from the body of the cell and perform their functions effectively. Flagella are longer in size, which grows up to 150 um in length. Flagella are found in both prokaryotic and eukaryotic cell and serve the same functions in both types of cells but are structurally different. The majority of bacteria possess flagella that are used for locomotion.

Cilia (singular = cilium) are microscopic, hair-like, slender, and membrane-bound structures that extend from the surface of many eukaryotic cells. They are short in length(5-20 um)  and perform to move entire cells or move substances around cells. Cilia of the fallopian tubes help to move the ovum towards the uterus and cilia of the respiratory tract move particulate matter toward the throat.

Where Found

Cilia are found in:

  • Ciliate protozoans, such as Paramacium
  • Flame cells of flatworms
  • Many larval forms of invertebrates such as bipinnaria larva, tornaria larva, Trochophore larva, Amphilinidea larva, etc.
  • Epithelium of respiratory tract
  • Fallopian tube
  • Renal tubules
  • Fern sperms
  • Cycad sperms
  • Various gametes of slime molds, fungi, and animals.
image of Paramacium

Paramacium Shows Cilia 

Flagella are found in:

  • All flagellate protozoans such as Euglena, Clamydomonas, Phacus, etc.
  • Choanocytes of sponges
  • Gastrodermal cells of Cnidaria
  • Sperms of animals.
  • Sperms of bryophytes and pteridophytes
  • Gametes of many algae, etc.
image of Euglena

Euglena shows Flagellum

Parts of Ciliary Apparatus

The ciliary apparatus is composed of the following parts:

The shaft: It is the slender cylindrical process that projects from the surface of the cell. It lies external to the cell. In cilia, it is short (3-10 µm), but in flagella, it is longer (up to 150 µm). The axis of cilium consists of 2 central microtubules surrounded by nine pairs of microtubules (9+2) pattern), which is embedded in the ciliary matrix. It is covered by the ciliary membrane, which is the extension of the cell membrane. The basic microtubular framework, i.e., the axis of cilia and flagella, is called axoneme.

The basal body  or granule: The basal body or granule is a centriole-like cellular organelle that is arranged in rows beneath the cell membrane. From this, the body of the cilium originates. The basal body is also known as kinestosome.

Basal Plate: It is a plate-like structure which is located between the basal body and the shaft. In this region, the central singlet fibrils develop.

The ciliary rootlets: These are third components consisting of fine fibrils or ciliary rootlets, which arise from the basal body and converge to form conical bundles that end near the nucleus.

image of Structure and parts of a Flagellum

Structure and Parts of a Flagellum

Types of Cilia and Flagella

There are two types of cilia, such as kinocilia and Stericilia. In this case, kinocilia are motile that have the axonema whereas the steriocilia are non-motile and lack the axonema.

Besides these, Flagella are also of two types, such as tinsel flagellum and whiplash flagellum. In this case, the tinsel flagellum contains hairy outgrowths known as mastigonemes. On the other hand, whiplash flagellum does not contain any hair-like outgrowth.

Bacterial Flagella

In the bacterial cell, the following four types of flagella are found:

Monotrichous: This type of flagellum is found in Vibrio cholera.

Amphitrichous. This flagellum is found in Alkaligens faecalis. It originates from both ends of the bacterium body.

Lophotrichous: They originate from one or both sides of the bacterium body, such as Spirillum.

Peritrichous: They originate from all over the bacterial body and numerous in numbers such as Salmonella Typhi.

image of Bacteria based on flagellum

Bacteria showing flagella

Functions of Cilia and Flagella

  • Flagellum acts as the locomotory organs in many algae and other protists.
  • In the aquatic medium, the flagellum of many organisms helps to capture food particles.
  • Flagellum circulates food in the gastro-vascular cavity in coelenterates or spongocoel (choanocytes) in sponges.
  • The cilia lining in the cells of the respiratory tract remove solid or dust particles from it.
  • In many aquatic organisms, cilia can create water current to receive oxygen (O2) and quick remove of CO2.
  • Cilia perform many essential tasks such as they help to the passage of eggs in the oviduct, passage of excretory substances in the kidneys, etc.
  • In many cases, cilia and flagella perform sensory functions such as sensitivity to changes in light, temperature, and contact, etc.

Difference Between Cilia and Flagella

Cillia

Flagella

Cilia are numerous in number.

Flagella are less (usually 1-2) in number.

Cilia may occur throughout the surface of the cell.

Flagella occur at one end of the cell.

The cilia are shorter in length(5-20µ).

The flagella are longer in size(150µ).

he movement of cilia takes place in coordinated rhythm.

Flagella move independently.

Cilia move in a sweeping or pendular stroke.

Flagella exhibit undulatory motion.

In cross section, Nexin arm is present.

In this case, Nexin arm is absent.

It is found in eukaryotic cells.

It is found in both eukaryotic and prokaryotic cell.

For performing the movement, they use kinesin which has an ATPase activity that produces energy.

Flagella perform functions powered by proton-motive force by the plasma membrane.

Cilia help in locomotion, feeding, aeration, etc.

It helps only in locomotion.

Microtubules: Structure and Functions

The cytoplasm of the eukaryotic cell contains numerous hollow ultrafine non-membranous tubules made up of tubulin protein and involved in the movement, and determination of cell shape is known as microtubules. Besides tubulin, microtubules also contain many other proteins, such as kinesin and dynein. The combination of microtubules and microfilaments forms the cytoskeleton of the cell. Microtubules are responsible for the movement of the cell membrane, organelles, and cytoplasm.

image of Microtubule

The electron microscope has revealed that the cytoplasmic matrix of most eukaryotic cells contains microtubule and micro-filaments. In addition to this, the microtubules also occur in cilia, flagella, centrioles, and basal bodies, etc. and microfilaments are found in the axons of the neurons.

Microtubules are polymers of tubulin that have fine hollow cylinders of variable length. Generally, the microtubule consists of 13 protofilaments in the tubular arrangement. In some bacterial, microtubule consists of a ring of five protofilaments.

A microtubule can grow up to 50 µm in length and is highly dynamic. Tubules are very stiff in nature, and the outer diameter of a microtubule ranges from 23- 27 nm while the inner diameter from11-15 nm. The wall of the microtubule is about 60 Å in thickness and made up of alternating helix of α-tubulin and β-tubulin. Tubulin is a globular protein in nature. The α- and β-tubulins combine and form one dimer. Electron microscopic structure of cells has revealed that spindle fibers are created by the aggregation of much smaller fibers called microtubules.

Functions of Microtubules

  • They form a supporting framework or cytoskeleton and give shape to the cell.
  • They form the mitotic apparatus (spindles) consisting of bundles of microtubules.
  • They help to make up the internal structure of flagella and cilia.
  • They are related to the movement, such as the undulation of cilia and flagella.
  • Microtubules are involved in the transport of macromolecules within the cells.
  • During cell differentiation, cells change their shapes with the help of microtubules.
  • They also take part in cell division. In this case, they make up the mitotic spindles as the major constituents.
  • They are involved in various cellular processes, such as the movement of the secretory vesicles and organelles.

Cytoplasmic Inclusions

The contents of the cell between the plasma membrane and nuclear envelope are known as cytoplasm. It is a gel-like clear substance that contains living and non-living materials such as water, enzymes, salts, organelles, and various organic molecules. Among them, organelles form the living inclusions, and non-living substances form cytoplasmic inclusions. These substances do not possess the metabolic activity and are not bounded by membranes. It is also known as ergastic substances. O. F. Müller first observed these structures in 1786.

The cytoplasmic inclusions or non-living cell contents may be classified into the following three main groups:

  • 1. Reserved Products
  • 2. Secretory Products
  • 3. Excretory Products

Reserved Products

The reserved products are formed by various metabolic activities of the cells. These materials are produced and stored in that particular cell. The reserved products are of the following three main groups:

  1. Carbohydrate
  2. Proteins
  3. Fats or oils

Carbohydrates

Carbohydrates are neutral organic compounds that are made up of carbon(C), hydrogen (H), and oxygen (O) ions. Hydrogen and oxygen remain in the same proportion as in water that is 2:1 (H2O). Carbohydrates are classified into three types, namely monosaccharide, which contains one molecule of sugar, oligosaccharide, which contains 2-10 molecules of sugars and polysaccharide, which contains more than ten molecules of sugars. Glucose, fructose, galactose, etc. are monosaccharides; maltose, lactose, sucrose are disaccharides (oligosaccharide). These monosaccharides and disaccharides are all soluble sugars that remain dissolved in the cytoplasm. Starch, cellulose, and hemicelluloses are insoluble polysaccharides found in plant cells. Glycogen is a soluble polysaccharide found in animal cells. 

image of different Sugar

Starch (C6H10O5)n: In the cytoplasm, starch remains scattered in the form of particles. These particles are called starch grains. They may be round, oval (potato), polygonal(maize) or spherical(pea) in shape. The grains are stratified because starch is deposited in layers around a definite point known as hilum. When the layers are deposited in such a way that the hilum remains on one side as in potato, the grain is said to be eccentric when the hilum remains at the center as in pea; the grain is said to be concentric. Starch grain may be simple when they occur singly or compound when they occur together in a solid group.

image of Starch
image of Different type starch grain

Image showing different types starch grains

Glycogen (C6H10O5)n: It is a multi-branched polysaccharide that is mainly found in the liver and muscle cells of the animal body. Hence, it is also known as animal starch. Generally, it is present in the cytoplasm in between mitochondria and endoplasmic reticulum in the form of particles. In the liver cells, two types of particles are found, such as smaller β-particles and larger rosette-like α-particles. It is also found in blue-green algae, fungi, and in some lower plants.

image of Chemical structure of glycogen

Chemical structure Glycogen

Inulin (C6H10O5)n: It is a polysaccharide carbohydrate that is found in many roots of composite plants. It yields fructose on hydrolysis. It is colorless, tasteless, white amorphous powder or crystals and soluble in water.

Protein

It is an organic compound which is generally made up of carbon(C), hydrogen (H), oxygen (O) and nitrogen (N), sometimes it also contains sulfur (S) and phosphorus(P). These elements form amino acids, the unit of protein. Proteins are classified into three groups, namely pure protein, conjugated protein, and derived protein. The cell membrane, protoplasm, nucleoplasm, and the cytoplasmic organelles are made up of protein. Some proteins are soluble in water, and some are insoluble in water. In insoluble protein, particles remain scattered within the cytoplasm, which is known as proteid grains.

In certain plants, proteid grains are called aleurone grains. They are found to occur in several seeds. Aleurone grains are larger and are found in those seeds that contain less starch, as in castor. These grains are relatively small and are found in those seeds that contain abundant starch, as in maize. It is oval or spherical in shape and consists of two parts, namely a proteinaceous crystal-like polygonal body, called crystalloid, and a rounded mineral body, called globoid. The smaller grains are devoid of globoid.

image of Aleurone grain

Image showing aleurone grain (castor bean)

Fats or oils

Fats are an ester of fatty acids and glycerol. Both are made up of carbon, hydrogen, and oxygen. At room temperature, some fats remain solid and others in liquid forms, which are known as oils. In animals, fats are mostly stored in fat cells of connective tissue. The big fat droplets occupy almost the whole of the cell. The cytoplasm and nucleus are pushed out to one side. In plants, fats are generally stored in the seeds.

Secretory Products

Various products like nectar, coloring material (pigments), hormones and enzymes, etc. which are secreted by the cells, are called secretory products.

Nectar: It is secreted by a special type of glands, called nectaries, present in many flowers. It attracts the insects for pollination. 

image of Nectar secreting cells

Image showing nectar secreting cells

Coloring materials: These are produced by the plant cell. The coloring materials include chloroplasts, chromoplasts, and anthocyanins, etc. These are essentials in the process of photosynthesis. They also provide coloration to various organs of the plant.

RBC of the animal contains hemoglobin.  It is an iron-containing pigment. Melanin is a brown to black pigment present in the skin, eye, etc.

Hormones and enzymes: These are organic compounds that are secreted by the animal and plant cells. They have a profound influence on the metabolism, growth, and development of animal and plant bodies.

Zymogen granules:  Zymogen granules are inactive precursors of some digestive enzymes. They are secreted from glandular cells. They are large, spherical, and homogeneously dense granules with a single membrane. These granules are present in the cytoplasm in between Golgi bodies and the free surface of the cell. Its average diameter is 0.1-0.5 µ and generally found in the secretory cells like acinar cells of the pancreas and chief cells of the stomach. In the pancreatic acinar cells, protein is first synthesized directly into the cisternae of the endoplasmic reticulum(ER). Zymogen accumulates in the Golgi bodies, where it is concentrated and forms granules. This process is known as packaging. Zymogen granules then bud off as secretory granules. They are transported to the cell membrane and eliminate them into the external environment.

Excretory Products

Various harmful products are formed in the cell due to metabolism.  These are not secreted but stored in the cytoplasm of the cell. These are known as excretory products of the cell. These products are useful to mankind.

Mineral crystals

The common forms of crystal secreted by plant cells are made up of silica, calcium carbonate, and calcium oxalate. Calcium oxalate is the most common and is widely distributed among the various plants.

(i) Cystolith: It is an outgrowth of the epidermal cell wall; Calcium carbonate occurs as a large mass of small crystals in many plants leaves. The whole crystalline mass looks like a bunch of grapes suspended from a stalk from the upper epidermis. It is known as cystolith. Cystoliths are found in certain plant families, such as Acanthaceae, Urticaceae, Cannabaceae, Moraceae (Ficus elastica), etc. 

image of Cystoliths

Image showing systolith

(ii) Raphides: Mainly calcium oxalate occurs as aggregates of crystals in the form of raphides. The bag-like specialized cells where these are formed are known as idioblasts. These may be seen singly or in a bundle. The crystals are needle-like structures. They are commonly seen in water hyacinth (Eichhornia crassipes), Colocasia (Araceae), leaves of Bougainvillea (Nyctaginaceae), and in many other plants.

image of Raphides

Image showing Raphides

Tannins

These are a group of complex compounds; it commonly occurs in single isolated cells. These are abundant in the bark caves and many unripe fruits. Tea leaves contain about 15% tannins.

Essentials oils

These are present in oil glands, which are found in the leaves of sacred basil or tulsi (Ocimum tenuiflorum), lemon, petals of the rose, etc. The common essential oils are sandalwood oil, rose oil, and clove oil.

Resins

Resins are yellowish solids, insoluble in water, but soluble in alcohol. They are found in the stems of conifers and sal trees ( Shorea robusta).

Gums

It is an amorphous colloid and consists largely of decomposition products of cellulose or other carbohydrates of the cell wall. They occur in mixtures with resins and complex carbohydrates. Common examples are camphor(Cinnamomum camphora),  goggul(Commiphora wightii), gum Arabic(Acacia species), gum tragocanth ( Astragalus), etc.

Latex

It is generally milky, viscous colloidal substances, found in latex cells and latex vessels. It is often found in Indian rubber plant (Ficus elastica), para rubber tree (Hevea brasiliensis) and calotropis (milkweeds: Apocynaceae). It is also found in opium poppy (Papaver somniferum).

image of Latex vessels

Image showing Latex vessels

Alkaloids

These are complex nitrogenous substances and are present in the seeds and roots of some plants. These are intensely bitter, and some of them are poisonous. The caffeine of coffee and tea, nicotine of tobacco, quinine of cinchona(Rubiaceae), morphine of opium poppy, and strychnine of Strychnine tree (Strychnos nux-vomica) are the best examples of alkaloids.

Some Functions of Cytoplasmic Inclusions

  • These substances aid the organism in defense.
  • It does maintenance of the cellular structure.
  • It helps to store various materials. 
  • Some inclusions such as hormones, enzymes, etc. influence the metabolism, growth, and development of animal and plant bodies.
  • Inclusions like nectar attract the insects for pollination.
  • Tannins of plants play a role in protection from predation and might help in regulating plant growth.
  • The resin of plants protects the plant from pathogens and insects.

Concluding Remarks

The cytoplasm is a gel-like, highly viscous substance that is composed of three types of structure, including the cytoplasmic matrix, the cytoplasmic organelles, and the cytoplasmic inclusions. They are enclosed within the cell membrane, and they contain about 85% water, 10-15% proteins, 2-4% lipids, inorganic salts, nucleic acids, and carbohydrates in a small amount. Most of the metabolic activities occur within the cytoplasm. During metabolic activities, some non-living substances such as nutrients, pigments, hormones, enzymes, etc. are produced. These products form the cytoplasmic inclusions and perform various functions.

Cell Related Terms and Definitions

  • Biology: Biology is a branch of science which deals with the study of living organisms.
  • Cytology: Cytology or cell biology is the study of the structure and function of the cell.
  • Cell: Cell is the functional and fundamental unit of life.
  • Cellular Respiration: Cellular respiration is a process by which cells produce the energy that is stored in food.
  • Cell Cycle: The cell cycle is the process of cell duplication and division, which includes interphase and mitotic cell division.
  • Plasma membrane: Plasma membrane is the semi-permeable lipoprotein covering that act as a selective barrier between the cell`s cytoplasm and its outside environment which protects the cell from its environment. It is also known as the cell membrane, cytoplasmic membrane or plasmalemma.
  • Mycoplasma: Mycoplasma is the simplest and smallest cellular organism.
  • Prokaryote: Prokaryote is the organism or cell (usually unicellular) which lacks a true nucleus.
  • Eukaryote: Eukaryote is the organism which contains a true nucleus.
  • Nucleoid: Nucleiod is the nuclear region within a prokaryotic cell that contains all or most of the genetic material(genophore) but not isolated by the membrane.
  • Endocytosis: Endocytosis is the process by which substances are brought into the cell from their external environment through the cell membrane. In this case, substances include fluids, proteins, electrolytes, and other macromolecules.
  • Exocytosis: Exocytosis is the energy-consuming process that forces out materials from within a cell to the extracellular space through the active transport by fusing with the cell membrane.
  • Phagocytosis: Phagocytosis is the process by which a cell ingests or engulf particulate material or other cells using its plasma membrane.
  • Pinocytosis: Pinocytosis is the process by which a cell ingests or absorbs of proteins and other soluble materials from outside the cell and brings them within the cell.
  • Desmosis: Desmosis is the thickened area of plasma membranes of two adjacent cells from which radiates out fine tonofibrils.
  • Cell wall: Cell wall is the rigid exoskeletal structure which encloses and protects the contents of most plant and some bacterial cell.
  • Plasmodesmata: Plasmadesmate is the cytoplasmic bridge between adjacent plant cells.
  • Protoplasm: Protoplasm is the ‘physical basis of life’.
  • Cytoplasm: Cytoplasm is that portion of cellular protoplasm which occurs between the plasma membrane and nuclear membrane.
  • Organelles: Organelles are the structures in the cytoplasm of a cell which take an active part in the life and function of the cell, for example, chloroplast.
  • Endoplasmic reticulum: Endoplasmic reticulum is the interconnected system of membrane-bounded tubules and vesicles which form irregular reticulum or network in the cytoplasmic matrix.
  • Golgi body: Golgi body is the cytoplasmic organelle which bears a complex system of tubules and vesicles and which is related to the production of cellular secretion.
  • Lysosome: Lysosome is the single membrane-bounded organelle which contains many kinds of hydrolytic enzymes which are involved intracellular digestion.
  • Suicidal Bag: Lysosome is also known as a suicidal bag.
  • Vacuole: Vacuole is the fluid filed cytoplasmic structure for storage of excess water, waste products, soluble pigments, etc.
  • Residual Body: Residual body is the vacuole containing indigestible substances.
  • Primary Lysosome: Primary lysosome is the small storage vesicle formed from the endoplasmic reticulum (ER).
  • Heterophagosome: It is the digestive vacuole resulting from the ingestion of foreign substances by the cell.
  • Autophagosome: Autophagosome is the specialized lysosome with a double-layer structure containing some parts of the cell during the process of digestion.
  • Mitochondria: Mitochondria is the membrane-bounded organelle that generates chemical energy in the form of ATP. It is also known as the powerhouse of the cell.
  • Crista: Crista (cristae: plural) is the infolding part of the inner mitochondrial membrane.
  • F1-particle: It is the membrane-bounded structure which is located on the matrix side of the cristae. It is also known as Fernandez Moran subunit.
  • Plastid: Plastid is the cytoplasmic organelle of the plant cell which is involved in synthesis and storage of starch, pigments and other products in plant cells.
  • Chloroplast: Chloroplast is the chlorophyll-containing plastid where photosynthesis occurs.
  • Thylakoid: Thylakoid is the flattened vesicle found in the chloroplast where the light-dependent reactions of photosynthesis take place.
  • Granum: Granum is the stalk of the thylakoid.
  • Chromoplast: Chromoplast is the yellow-orange coloured plastid.
  • Ribosome: Ribosome is the cytoplasmic granule composed of RNA and protein, where protein synthesis takes place.
  • Microtubules: Microtubules are the fine tubular structure which is made up of tubulin protein.
  • Cilia: Cilia (Cilium: singular) are the minute hair-like projections which occur from the surface of certain cells such as Paramecium.
  • Flagellum: Flagellum (flagella: plural) is the whip-like extended structure of the cell surface.
  • Basal body: Basal body is the cylindrical structure which is located at the base of each flagellum or cilium. It is also known as kinetosome.
  • Centriole: Centriole is the hollow cylindrical microtubular organelle which is involved in the organization of the spindle.
  • Cytoskeleton: Cytoskeleton is a complex cytoplasmic network of microtubules and microfilaments, which is located frequently near the cell membrane.
  • Nucleus: Nucleus is a part of a cell containing chromosomes; it controls the cell division and metabolism.
  • Karyotheca: Karyotheca is the double-membrane surrounding the nucleoplasm and genetic material.
  • Karyolymph: Karyolymph is the clear fluid material within the nuclear membrane.
  • Nucleolus: Nucleolus is the spheroidal body within the nucleus of a eukaryotic cell that makes the ribosomal subunits from proteins and rRNA. ( ribosomal RNA).
  • Chromatin: Chromatin is the genetic material (DNA and proteins) within the nucleus of a eukaryotic cell which forms a chromosome.
  • Cell Theory: Cell theory is one of the basic principles of biology, stating that the cell is the basic unit of life, and all living organisms are made from cells.
  • Centromere: Centromere is a region of the chromosome that joins two sister chromatids which plays an essential role in proper chromosome segregation during the cell division.
  • Chromatid: Chromatid is a replicated chromosome with two identical daughter strands attached by a single centromere. In this case, the two strands are separated to form individual chromosome during the cell division.
  • Chromosome: Chromosome is a long, rod-shaped or thread-like structure that carries heredity information (DNA) which is found in the nucleus of the cell.
  • Cytokinesis: Cytokinesis is a part of cell division where cytoplasm of a single eukaryotic cell is divided to produce two distinct daughter cells.
  • Cytosol: Cytosol is a semi-fluid substance of a cytoplasm found inside of cells in which organelles, proteins, and other cell structures float. It is also known as a cytoplasmic matrix or intracellular fluid, or groundplasm.
  • Organelles: Organelles are tiny specialized structures within a cell( such as mitochondria, ribosomes, endoplasmic reticulum, etc.), that carry out specific functions such as controlling cell growth and producing energy, which is necessary for regular cellular operation.
  • Peroxisome: Peroxisome is a membrane-bounded organelle which is found in the eukaryotic cell that contains enzymes involved in a variety of metabolic reactions and produces hydrogen peroxide as a by-product.

DNA : Its Structure and Functions

DNA or deoxyribonucleic acid is the double-stranded helical molecule in which genetic information is encoded as a sequence of purine and pyrimidine bases, attached pairwise by hydrogen bonds and longitudinally by sugar-phosphate backbones. German biochemist Frederich Miescher first observed DNA in 1869. He termed the material nuclein, which he isolated from pus cells that he collected from bandages discarded by a nearby clinic. Frederick Griffith realized that DNA might hold genetic information in 1928. Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin figured out the double helix structure of DNA in 1953. For their discoveries, Watson, Crick, and Wilkins rewarded the Nobel Prize in Medicine in 1962.

DNA is the main chemical structure of chromosomes of eukaryotic cells. It is also found in mitochondria and chloroplast in less quantity of eukaryotic cells, the cytoplasm of prokaryotic cells, and in some viruses. In eukaryotic cells, it remains combined with protein to form nucleoprotein.

Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are known as nucleic acid which are found in all living organisms. They are large molecules called macromolecules or polymers. They consist of repeating structural subunits monomer or nucleotides. Nucleotides are the building blocks of DNA, which consists of three parts: a pentose sugar (5-carbon sugar), a phosphate group, and a nitrogenous base.

Pentose Sugar

It is a monosaccharide that contains five carbons. Pentose sugars are of two types, such as ribose (in RNA) and deoxyribose (in DNA). DNA contains 2-β-D type deoxyribose pentose sugar in which an oxygen atom is lacking in carbon two positions of pentose structure.

image of Pentose sugar

Phosphoric Acid

A phosphate group is attached to carbon at 3′ of one pentose sugar and carbon atom 5′ of another pentose sugar. In this case, the phosphate group of one nucleotide links covalently with the sugar molecule of the next nucleotide. In this way, they form a long polymer of nucleotide monomers.

Nitrogenous Bases

Four types of nitrogenous bases are found in DNA. These are Adenine (A), guanine (G),  cytosine (C), and thymine (T). Among them, Adenine (A), guanine (G)  are collectively called purine bases, while cytosine (C) and thymine (T) are called pyrimidine bases. Here pyrimidine bases (C4H4N2) are single-ringed compounds, and purine bases (C5H4N4) are double fused ring compounds.

image of Nitrogenosu bases

Nucleotide

The nucleotide is a compound that is composed of three parts: a 5′ carbon pentose sugar molecule, nitrogenous base, and phosphate group. In this case, pentose sugar can be either ribose or a deoxyribose. Ribonucleotides or ribotides are the ribose containing nucleotide, while deoxyribonucleotides or deoxyribotides are the deoxyribose containing nucleotides.  In a nucleotide molecule, a nitrogenous base and a phosphate group attached to a pentose sugar. In this case, one to three phosphate groups can be attached to the 5′ carbon of the pentose sugar. The purine (adenine and guanine) and pyrimidine (cytosine, uracil, and thymine) are the nitrogenous bases.

Nucleotide = Sugar + Base + Phosphate

The Biological Functions of Nucleotides

  • Nucleotide makes up the building blocks of life.
  • It forms many different molecules that perform to make life possible.
  • It acts as storage of genetic information, as part of RNA or DNA.
  • It makes cellular communication.
  • It helps in co-enzyme catalysis.
  • It acts as messengers and energy moving molecules.
  • In living organisms, it makes up the genetic material.
  • It is involved in the synthesis of the polysaccharide.
  • It also performs cell signaling, metabolism, and enzyme reactions.

Nucleoside

A nucleoside is made up of a pentose sugar molecule and a nitrogenous base. It does not contain any phosphate group. A nitrogenous base is covalently attached to a pentose sugar, which can be either ribose or deoxyribose. Ribose containing nucleosides are called ribonucleosides or ribosides, while deoxyribose containing nucleosides are called deoxyribonucleosides or deoxyribosides. In the case of nucleoside, nitrogenous bases and the pentose sugars are the same as in the nucleotide.

Nucleoside = Sugar + Nitrogenous base

Examples of nucleosides: Adenosine, thymidine, uridine, guanosine, cytidine etc.

Functions of Nucleosides

  • It is a structural subunit of nucleic acids.
  • It is heredity controlling components of all living cells.

The following table shows the examples of nucleosides and nucleotides with corresponding nitrogenous bases.

Nucleic Acid

Nitrogen Base

Nucleoside

Sugar + Base

Nucleotide

Sugar+Base+Phosphate

RNA

Adenine(A)

Adenosine

Adenosine monophosphate(AMP)

RNA

Guanine(G)

Guanosine

Guanosine monophosphate(GMP)

RNA

Cytosine(C)

Cytidine

Cytidine monophosphate(CMP)

RNA

Uracil (U)

Uridine

Uridine monophosphate(UMP)

DNA

Adenine(A)

Deoxyadenosine

Deoxyadenosine monophosphate(dAMP)

DNA

Guanine(G)

Deoxyguanosine

Deoxyguanosine monophosphate (dGMP)

DNA

Cytosine(C)

Deoxycytidine

Deoxycytidine monophosphate(dCMP)

DNA

Thymine(T)

Deoxythymidine

Deoxythymidine monophosphate (dTMP)

Structure of DNA

image of Structure of DNA
  • Wilkin, Franklin, Watson, and Crick proposed a model for the DNA structure in 1953, based on the X-ray diffraction data.
  • It is composed of two anti-parallel polynucleotide chains (strands) that form a double helix around a central axis. It looks like a ladder.
  • Deoxyribose and phosphate groups alternately make the backbone of each stand or polynucleotide chain.
  • The two strands are inter-twisted in a clockwise direction. In the two antiparallel strands, one strand runs 5′ to 3′ direction while the other runs 3′ to 5′ direction.
  • By making a covalent bond, the nucleotides are attached in a polynucleotide chain between the pentose sugar of one nucleotide and the phosphate of the next nucleotide, which form an alternating sugar-phosphate backbone.
  • The two strands are linked together by hydrogen bonds established between the pairs of the purine or pyrimidine bases. 
  • According to base-pairing rules, nitrogenous bases are linked together to make the double-stranded DNA molecule.
  • Adenine always connected with thymine by two hydrogen bonds (A=T), and Guanine is connected with cytosine by three hydrogen bonds (G≡C).
  • The helix makes one complete turn in every 340 in just over ten nucleotide pairs and has a diameter of about 20Å.
  • Along one polynucleotide chain the axial sequence of bases may vary considerably, but the sequence must be complementary in the other chain. These sequences are given away in the following examples
image of Nitrogen base sequence

Functions of DNA

  • DNA is necessary for the production of proteins.
  • It helps to regulate the metabolism and reproduction of the cell.
  • It holds all of the genetic information.
  • DNA produces characteristics of an individual and species.
  • DNA plays a vital role in the replication of DNA hence increase in the number of chromosomes and cells.
  • It helps the formation of RNA.
  • DNA helps in the exchange of genetic information from parents to progeny.

Difference between Nucleoside and Nucleotide

Nucleoside

Nucleotide

It is made up of a pentose sugar and nitrogenous base.

It is made up of pentose sugar, a phosphate group and a nitrogenous base.

It is the precursor of nucleotide.

It is the precursor of polynucleotides, DNA and RNA

Rich nucleosides` diet makes the optimum health. 

It is used in signal transduction pathways, sequencing and as an energy source.

There are several nucleoside analogues which are used in therapeutic drugs such as antiviral or anticancer agents to prevent viral replication in infected cells.

Locked nucleic acid (LNA), Peptide nucleic acid (PNA), etc are the analogous for the sugar backbone in RNA which regulate the gene expression.

Examples: Adenosine, thymidine, uridine, guanosine, cytidine, etc.

Examples: AMP(Adenine monophosphate), ADP (adenine diphosphate), and ATP (adenine triphosphate), etc.

Difference between Purine and Pyrimidine

Purines

Pyrimidines

It is made up of  two hydrogen-carbon rings and four nitrogen atoms.

It is made up of one hydrogen-carbon ring and two nitrogen atoms

Nucleonbases are adenine and guanine.

Nucleonbases are Cytosine, thymine, and uracil.

It is bigger in size.

It is smaller in size.

The melting Point of purine is  214 °C (417 °F).

The melting point of pyrimidine is 20 to 22 °C (68 to 72 °F)

It`s chemical formula is C5H4N4.

It`s chemical formula is C4H4N2.

It is biosynthesized in liver.

It is biosynthesized in various tissues.

Catabolism Product of purine is uric acid (C5H4N4O3).

Catabolism Products are ammonia (NH3) and carbon dioxide (CO2).

It`s molar mass is 120.11 g mol−1.

It`s molar mass is 80.088 g mol-1.

It produces DNA and RNA.

It is used in storage of energy.

It synthesizes protein and starch.

It can perform cell signaling.

It also helps in enzyme regulation.

It produces DNA and RNA.

It is used in storage of energy.

It synthesizes protein and starch.

It can perform cell signaling.

It also helps in enzyme regulation.

Concluding Remarks

DNA or Deoxyribonucleic acid is the double-stranded helical molecule that contains genetic information. Swiss physician Friedrich Miescher first isolated the DNA molecule from the pus cells of discarded surgical bandages in 1869. Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin figured out the double helix structure of DNA in 1953. Generally, DNA occurs as linear chromosomes and circular chromosomes in eukaryotic and prokaryotic cells, respectively. Each genome is made by the set of chromosomes in a cell.  There are approximately three billion base pairs of DNA in the human genome, which is arranged into 23 pairs of chromosomes.   DNA carries and transmits the genetic information which is achieved via complementary base pairing.

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