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:
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:
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.
It is the most important and longest stage closely similar to the early mitotic prophase. It is divided into the following sub-stages:
- Diplotene and
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:
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:
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::
Diplotene: The stage of meiosis in which paired homologues begin to separate is known as diplotene. The characteristic features of diplotene are:
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 characteristic features of prometaphase are:
The characteristic features of metaphase-I are:
The characteristic features of anaphase-I are:
The characteristic features are the following:
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.
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:
The characteristic features of prophase-II are:
The characteristic features of metaphase-II are:
The characteristic features of Anaphase-II are:
The characteristic features of Telophase-II are:
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.
Significance of Meiosis
Difference Between Mitosis and 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.
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.
Spindle fibers disappear completely in telophase,
Spindle fibers disappear completely during telophase-I.
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,
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.
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.