Question

A DNA molecule is 140 base pairs long. It has 20% thymine. How many guanine bases are present in the molecule?

Options

• 64

• 84

• 32

• 56

Answer 1

84

Explanation:

By Chargaff’s rules

A = T and

C = G,

so if T = 20% then A = 20%

(A + T = 40%), 

leaving C + G = 60% and thus G = 30% of all bases.

A 140 bp double‑stranded DNA has 280 bases total, so

G = 0.30 × 280

= 84.