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.

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.

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

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

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 C₅H₄N_4.	It`s chemical formula is C₄H₄N_2.
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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