Question

The amount of guanine in a sample of DNA sequence is found to be 32%. Which of the following is true for the composition of DNA sample?

• A. It consists of 18% cytosine and 18% thymine.
• B. It consists of 18% adenine and 18% cytosine.
• C. It consists of 32% cytosine and 18% thymine.
• D. It consists of 32% adenine and 18% cytosine.
• E. It consists of 32% thymine and 18% adenine.

Answer 1

The correct option is C It consists of 32% cytosine and 18% thymine.

According to Chargaff’s rule, in all cellular DNAs, regardless of the species, number of adenosine residues is equal to number of thymidine residues which means that A = T; and the number of guanosine residues is equal to the number of cytidine residues; G = C. Hence that the sum of the purine residues equals the sum of the pyrimidine residues; i.e., A + G = T + C. Here [G] = 32% therefore, % of [C] is also 32%. Thus, [A] + [T] = 100 - 64 = 36 %. [A] = 18% and [T] = 18%.

Therefore, the correct answer is option C.