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The below mentioned article provides a diagram of DNA strand.
(a) DNA is made up of two polynucleotide chains. The backbone is constituted by sugar-phosphate, while the nitrogenous bases project inwards.
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(b) The two chains have anti-parallel polarity. Therefore, when one chain has 3′ → 5’polarity, the other chain has 5′ → 3′ polarity.
(c) The bases in two strands are paired through hydrogen bonds (H-bonds) to form base-pairs.
i. Adenine is bonded with thymine of the opposite strand with the help of two hydrogen bonds.
ii. Guanine is bonded with cytosine of the opposite strand with the help of three hydrogen bonds. So, a purine bonds with a pyrimidine always. This maintains a uniform distance between the two strands of the helix.
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(d) The two polypeptide chains are coiled in a right-handed fashion.
Pitch of the helix = 3.4 nm
Number of base pairs in each turn = 10
Distance between a base pair in a helix = 0.34 nm.
(e) The plane of one base pair stacks over the other in double helix. This provides the stability to the helical structure, in addition to H-bonds, which also provide stability. The length of DNA in E. coli is 1.36 mm, while in humans is 2.2 m.
