A scientist analysing a sample of DNA finds that 30% of its bases are adenine. Without measuring anything else, she can immediately predict that close to 30% of the bases in that sample will be one other specific base. Which base is it, and why?
- Thymine, because adenine always pairs with thymine across the two strands of the DNA double helix
- Guanine, because adenine and guanine are always present in exactly equal amounts in every organism's DNA
- Cytosine, because adenine and cytosine bond directly to each other to hold the two DNA strands together
- No other base can be predicted, because adenine pairs with a different base in every organism's DNA
Why A? And why not the others?
Correct answer: A. Thymine, because adenine always pairs with thymine across the two strands of the DNA double helix
DNA's double helix is held together by fixed complementary base pairing: adenine always pairs with thymine (forming two hydrogen bonds), and guanine always pairs with cytosine (forming three hydrogen bonds), so the amount of adenine in any sample of DNA, from any organism, always closely matches the amount of thymine. The option naming guanine is wrong because adenine does not pair with guanine at all - guanine's fixed partner is cytosine, not adenine, and the amounts of adenine and guanine are not linked to each other. The option naming cytosine is wrong for the same reason: cytosine's bonding partner across the helix is guanine, never adenine, so predicting cytosine's amount from adenine's amount has no basis. The option claiming no prediction is possible is wrong because this base-pairing rule (sometimes called Chargaff's rule) is one of the most reliable and universal patterns in molecular biology, holding true across essentially all double-stranded DNA regardless of species, which is exactly why the scientist can make her prediction with confidence.
Source: BBC Bitesize GCSE Biology: DNA structure and complementary base pairing (Chargaff's rule)