DNA STRUCTURE

The DNA structure consists of two strands wound around each other. Each strand of a DNA molecule is composed of a long chain monomer nucleotides. The nucleotides of DNA consists of a deoxyribose sugar molecule to which is attached a phosphate group and one of four nitrogenous bases: two purines (adenine and guanine) and two pyrimidines (cytosine and thymine). The nucleotides are joined together by covalent bonds between the phosphate of one nucleotide and the sugar of the next, forming a phosphate sugar backbone from which the nitrogenous bases protrude. One strand is held to another by hydrogen bonds between the bases; the sequencing of this bonding is specific ie adenine bonds to thymine and cytosine to guanine

The above image shows the DNA structure showing the nucleotides bases cytosine(C), thymine(T), adenine(A) and guanine(G) linked to a backbone of alternating phosphate and deoxyribose sugar (S) groups. Two sugar-phosphate chains are paired through hydrogen bonds between A and T and between G and C thus forming the twin stranded double helix of the DNA molecule.

The DNA coils up, forming chromosomes, and each chromosome has a single molecule of DNA in it. Overall, human beings have around twenty-three pairs of chromosomes in the nucleus of cells. DNA also plays an essential role in the process of cell division.

The image below shows a simplified version of the nucleotide pairing

The order of the nitrogenous bases determines the genetic code or the DNA’s instructions.

This image shows a simplified structure of the DNA compounds

Types Of DNA

1. A-DNA: It is found at a relative humidity of 75%. In an environment where there is a higher salt concentration or ionic concentrations, such as K+, Na+, Cs+ or in a state of dehydration it endures in a form that contains 11 nucleotide pairs with a rise of 2.56Å vertically per base pair. It has the broadest helical diameter amongst all DNA forms – 23Å DNA which is a typical helix that is right-handed with a rotation of 32.70 per base pair.

2. B-DNA: The most common form, present in most DNA at neutral pH and physiological salt concentrations, is B-form. It has 10 base pairs per turn from the helix axis. There is a distance of 3.4Å with a helical diameter of 20Å. Watson-Crick’s double helix model is defined as a B-form of DNA.

3. C-DNA: It is observed at a relative humidity of 66% and in the occupancy of a few ions such as Lithium(Li+). It closely has 9.33 base pairs for every turn. The diameter of the helix is about 19Å and the vertical rise for every base pair for the right-handed helix is 3.320.

4. D-DNA: It is observed rarely as an extreme variant. The 8 base pairs are titled negatively from the helix axis with an axial rise of about 3.03Å

5. Z-DNA: It is found in an environment with a very high salt concentration. Unlike the A, B, and C types of DNA, it is a left-handed helical structure. The backbone is arranged in a zig-zag pattern formed by the sugar-phosphate linkage wherein the recurrent monomer is the dinucleotide in contrast to the mononucleotide, which is observed in alternate forms.

DNA FUNCTIONS

Polypeptide chains are further folded in secondary, tertiary and quaternary structures to form different proteins. As every organism contains many genes in its DNA, different types of proteins can be formed.Proteins are the main functional and structural molecules in most organisms. Apart from storing genetic information, DNA is involved in:

• Replication process: Transferring the genetic information from one cell to its daughters and from one generation to the next and equal distribution of DNA during the cell division
• Mutations: The changes which occur in the DNA sequences
• Transcription
• Cellular Metabolism
• DNA Fingerprinting
• Gene Therapy