CBSE Class 10 Answered

Hi Riya,

The structure of DNA is detailed below:

• Deoxyribonucleic acid (DNA) is composed of two polynucleotide strands which form a ladder.
• The nitrogenous bases in DNA store the information for the synthesis of polypeptide chains, essentially coding for every feature of the entire organism.
• The two polynucleotide strands run 'antiparallel' (in opposite directions, parallel to one another) to each other, with nitrogenous bases projecting inwards.
• The antiparallel strands twist in a complete DNA structure, forming a double helix.
• The strands are held together by hydrogen bonds between the nitrogenous bases which lie opposite to each other.
• Bases bonded together are termed 'paired' and are very specific as to which base they will join.
• A purine will only pair with a pyrimidine. The purine adenine will always pair with the pyrimidine thymine (A–T). The purine guanine will only pair with the pyrimidine cytosine (G–C). These base pairings are termed complementary base pairings.
• Purines will always bond with pyrimidines because purines are larger molecules composed of a double ring structure. So, in order to ensure that the polynucleotide strands are equally spaced apart, the larger bases must pair with the smaller bases.
• The root for specific complementary base pairings is the number of hydrogen bonding sites available. Adenine and thymine have two sites each, whereas guanine and cytosine have three sites each.