The salient features of the Double-helical DNA structure proposed by James Watson and Francis Crick are given below:
(i) DNA is made of two polynucleotide chains where the sugar-phosphate forms the backbone, and the bases are present inside.
(ii) The two polynucleotide chains of DNA have anti-parallel polarity. Which means that if one chain has 5’→ 3′ polarity, then the other chain has 3′ → 5′ polarity.
(iii) The bases in two strands of DNA are paired with hydrogen bond forming base pairs (bp). Adenine forms two hydrogen bonds with Thymine and vice-versa in opposite direction. Similarly, Guanine forms three hydrogen bond with Cytosine. Thus, purines and pyrimidines are opposite to each other. This approximately maintains uniform distance between the two strands of the helix.
(iv) The two polynucleotide chains are coiled in a right-handed fashion and the pitch of the helix is 3.4 nm and there are 10 bp (base pairs) in each turn of a helix. The distance between a base pair in a helix is approximately 0.34 nm.
(v) The plane of one base pair always stacks over the other in a DNA double helix and this stacks in addition to hydrogen bonds, provides stability to the helical structure of DNA.
Structure of a Polynucleotide Chain
(i) A large number of nucleotides when joined together in a chain forms a polynucleotide chain.
(ii) All the nucleotides in a polynucleotide chain are joined by 3’-5’ phosphodiester bond.
(iii) A nucleotide contains three different components, these are: a nitrogenous base, a pentose sugar and a phosphate group.
(iv) There are again two different types of nitrogenous bases, these are: Purines and Pyrimidines.
(v) Purines are of two different types – Adenine and Guanine.
(vi) Pyrimidines are of three different types – Cytosine, Uracil and Thymine.
(vii) Cytosine is present in both DNA and RNA, but Thymine is present only in DNA and Uracil is present only in RNA (instead of Thymine).
(viii) A nitrogenous base joined to the OH of 1’ C (or carbon) of pentose sugar by N-glycosidic bond forms a nucleoside, such as: adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine.
(ix) A phosphate group joined to the OH of 5′ C (or carbon) of a nucleoside by phosphoester bond forms a corresponding nucleotide, such as: adenine, guanine, cytosine, uracil and thymine.
(x) Thus, a polymer formed has a phosphate moiety at 5′-end of ribose sugar at its one end and this end of a polynucleotide chain is called as the 5’-end.
While at the other end of a polymer, the ribose sugar has a free 3′-OH group and this end of a polynucleotide chain is called as the 3′-end.
Therefore, the backbone of a polynucleotide chain is made of pentose sugar and phosphate group.
And the nitrogenous bases when gets linked to sugar moiety forms the complete DNA structure.