Question

The ratio constant for a species is

• A. T + C/G + A
• B. A + G/C + T
• C. A + C/T + G
• D. G + C/A + T

Answer 1

The correct option is D G + C/A + T

A + G/C + T or T + C/G + A
According to the Chargaff’s rule, irrespective of the source of DNA, the ratio of total amount of purines and total amount of pyrimidines remains constant and equal in DNA, that is A + G = T + C. Hence the ratio is 1 for all species and not just for a particular species.

A + C/T + G
The percentage of adenine (A) is equal to the percentage of thymine (T) and the percentage of guanine (G) is equal to the percentage of cytosine (C) in double stranded DNA, that is A = T; G = C. Thus, ratio A + C/ T+G will be random even within the members of a species as A does not pair with C and T does not pair with G.

G + C/A + T
Chargaff stated that the amount of A+T is not necessarily equal to the amount of G+C. But the ratio of (A+T)/(G+C) or (G+C)/(A+T) is constant for a particular species and varies from one species to another.

Final answer
(D) G + C/A + T