Question

In a sample of double-stranded DNA, if adenine is 28 percent, what is the amount of cytosine present?

• A. 23 percent
• B. 25 percent
• C. 46 percent
• D. 22 percent

Hint:

A equals T and G equals C, and all four bases add up to 100 percent.

Answer 1

The Correct Option is D

This is a direct Chargaff's rule calculation. In any double-stranded DNA the two purine-pyrimidine pairings force adenine to equal thymine and guanine to equal cytosine, and the four percentages must total 100. Plug in the number: adenine is 28 percent, so thymine matches at 28 percent. Their sum is $28 + 28 = 56$ percent, which leaves $100 - 56 = 44$ percent for the guanine plus cytosine pair. Since guanine equals cytosine, split that evenly: $44 / 2 = 22$ percent each. So cytosine is 22 percent, option (d). A fast sanity check: cytosine and guanine are equal, and adenine and thymine are equal, and 28 plus 28 plus 22 plus 22 lands exactly on 100. Recall pearl: A-T pairs are held by two hydrogen bonds and G-C pairs by three, so a higher G-C content makes DNA more thermally stable, which is a frequent follow-up point.