Question

In the DNA of an animal, the percentage of adenine is 30 then percentage of guanine will be?

• A. 40
• B. 30
• C. 20
• D. 70

Answer 1

The correct option is C 20

DNA, i.e deoxyribonucleic acid, is a polymer of nucleotides. Erwin Chargaff proposed three rules which are termed as chargaff's rules. These rules played an important role in the discovery of the double helix structure of DNA, they are

1. In any double stranded DNA, the number of guanine units equals the number of cytosine units whereas the number of adenine units equals the number of thymine units (i.e no of A = no of T and no of C = no of G and vice versa ).

2. The sum of purines (A and G) equals that of pyrimidines (C and T). The composition of DNA varies from one species to another. Now as per the given question, the percentage of Adenine is 30. Thus, the percentage of Thymine should be 30 as per the first rule. The sum of A and T would be 60 %. Remaining % of bases is 40% which has to be divided equally between G and C since no of G= no of C. Thus G=C=20%.

So, the correct option is C.