Deoxyribonucleic Acid (DNA)
DNA is a genetic material which reserves genetic information (identity or character for traits). The DNA was first identified by Friedrich Mischer in 1869 who referred it as nuclein. Then James Watson and Crick in 1953 gave famous double helix model of DNA. Francis Crick gave central dogma which tells about the flow of genetic information from DNA to proteins.
DNA is a polynucleotide; a nucleotide is made up of nucleoside and phosphate group which are linked together by a phosphodiester linkage. A nucleoside is made up of a nitrogen base and pentose sugar by N-glycosidic linkage. Nitrogen bases are of two type purines (adenine, guanine) and pyrimidines (cytosine, uracil, thymine) and pentose sugar is deoxyribose. A dinucleotide made up of 3,5 phosphodiester linkage as phosphate group gets linked to 5-OH group. In this way, the polynucleotide is made.
DNA has a double helical structure; it is made up of 2 polynucleotide chains. Its backbone is composed of sugar phosphate with bases projected inward. It has anti-parallel polarity, one chain is 5’-3’, and another chain has polarity from 3’-5’. The chains are linked by hydrogen bonds which forms base pairs. In base pairing purine comes in opposite to pyrimidines, this leads to uniform separation in between strands of DNA. It is coiled in right-handed fashion, which creates 10 base pairs in each turn.
DNA is a large molecule which is packed to form chromosomes. It has a negative charge so it wraps around histone proteins octamer which is made of 8 molecules of histone proteins which form a nucleosome. Nucleosome repeated units form chromatin, chromatin gets further packed to form chromatin fibers and then again coiling and condensation of chromatin fibers occur to form chromosomes.