Question

If a double stranded DNA has 15% cytosine, what is the % of adenine in the DNA?
A)15%
B)70%
C)35%
D)30%

Answer 1

Hint: In a ds DNA molecule the four nitrogenous bases, namely Adenine, Guanine, Cytosine and Thymine form the backbone. Adenine pairs with Thymine via double hydrogen bonds while Guanine pairs with Cytosine via three hydrogen bonds.

Complete answer:
Watson and Crick suggested that DNA is composed of two strands that are twisted to form a right-handed helix around each other. The double helix of DNA is stabilized by two forces: hydrogen bonds formed in opposite strands and base-stacking interactions between paired bases.
Base pairing occurs between - A & T, C & G, to form base pairs as these bases are complementary to each other. Here, A and G are Purines while T and C are pyrimidines.
Chargaff determined that the amount of one base, purine, in DNA is often roughly equal to the amount of a specific second base, pyrimidine.
The amount of adenine(A) was very close to the amount of thymine(T) in individuals of the same genus, and the amount of cytosine(C) was very close to the amount of guanine(G), or A = T and G = C. These are called Chargaff's rules.
To calculate the percentage of Adenine in DNA we will use Chargaff’s rule;
Hence, if C = 15% then G = 15%,
Total GC % = 30%
A+T= 100-30 %
= 70%
Since A=T
Therefore, T = 35%, A = 35%

Let us analyze the options to find out the correct option.
Option A, B and D: These are incorrect options.
Option C: The % of adenine in the DNA is 35% according to Chargaff;s rule. Therefore, this is the correct option.

Hence, the correct answer is option (C)

Note: In 1947, Chargaff demonstrated that the composition of DNA changed from one species to another in terms of the relative amounts of the A, C, G and T bases. This molecular diversity provided a proof that the genetic material could be DNA.