What is DNA?

DNA( Deoxyribo Nucleic Acid)

DNA is mainly found in the nucleus. A small amount is laso found in mitochondria and chloroplasts. It is a macromolecule formed by the end to end polymerisation of a large number of repeated units called deoxyribonucleiotides  or simply nucleotides.

Wattson and Crick established the structure of DNA moleculein 1953 on the basis of the x-ray diffraction. For this excellent discovery, Wattson and Crick was awarded with Nobel Prize in 1962. According to them, DNA is composed of two antiparallel polynucleotide chains that form a double helix around the central axis.

The strands are made up of alternate bands of deoxyribose sugar and phospate molecules. They are joined by the phospdiester linkages.

Each deoxyribo sugar in the strand has one N-base horizonatally attached to it at carbon-1. The four N-bases can occur in any possible sequence along the length of a strand. The N-base + deoxyribose sugar + phosphate together form one unit or deoxyribonucleotide in the strand. Such many deoxyribonucleotides are linked with each other in a linear fashion, therefore, the ruslting strand is described as the polynucleotide strand and DNA moelucle as polynucleotide molecule.

The two polynucleotide strands are held together by hydrogen bonds between specific pairs of purines and pyrimidines. Purine of one polynucleotide chain pairs with pyrimidine of the other i.e. adenine (a) with thymine(T) and guanine (G) with cytosine (C). A and T are held together by two hydrogen bonds and G and C by three bonds. The sequence of bases in one polynucleotide chain automatically determines the order of bases in  the other i.i. two chains are complementary to each other. For example, where adenine occurs in one strand, thymine is present in the corresponding position position in the opposite strand and vice-versa. Similarly, wherever guanine is present in one strand, the other strand has cytosine opposite to it  and vice-versa.

The two strand of a helix are of opposite polarity. If one chain run in 3’-5’ direction then the other will run in opposite direction i.e. 5’-3’ direction.

Functions

1)      DNA is a genetic material. Hence it carries all the hereditary information from one generation to another generation.

2)      DNA has unique properties of formation of exact copies. This is essential for transfer of genetic information.

3)      DNA gives rise to RNA thorugh transcription process.

4)      DNA plays a key role in protein synthesis.

5)      Any change in the sequence of nitrogen bases of DNA causes mutaion.