Question

If the total amount of adenine and thymine in a double-stranded DNA is 60%, then the amount of guanine in this DNA will be

• A. 15%
• B. 20%
• C. 30%
• D. 40%

Answer 1

The correct option is B 20%

Purine (adenine and guanine) and pyrimidine (thymine and cytosine) nitrogenous bases are found in DNA.

Adenine pairs only with the thymine. Similarly guanine pairs with the cytosine.

The fact that the total amount of purine will be equal to the total amount of pyrimidine was first enunciated by Chargaff in 1950.

From Chargaff's law,

Adenine (A) = Thymine (T) = $\frac{60}{2}$ = 30%

Guanine (G) = Cytosine (C) = $\frac{40}{2}$ = 20%