Solution

By Chargaff's rule, in double-stranded DNA: A = T and G = C.
T = 20 percent, so A = 20 percent. A + T = 40 percent.
G + C = 100 $-$ 40 = 60 percent, and since G = C, G = 30 percent.
So the percentage of guanine is 30%.

Concept Base percentages in DNA

Chargaff's rules in past papers

3 questions from this chapter have appeared across 3 exam years.