If a DNA sample has 80% adenine and thymine, what percentage of nucleotides would be guanine and cytosine together?

• A. 60%
• B. 40%
• C. 20%
• D. 80%

Answer: (C)

Chargaff's rules tell us that in double-stranded DNA, adenine pairs with thymine (A = T) and guanine pairs with cytosine (G = C). So A+T and G+C together account for all nucleotides, totaling 100%. If A+T make up 80%, the remaining 20% must be G+C. Since G and C are equal in number, each would be 10%, but the question asks for the combined percentage of G and C, which is 20%.