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Nucleic Acids and Protein Synthesis: Question 1

Syllabus 6.1

Multiple choice AS 1 mark

In a molecule of DNA, which statement about the complementary base pairing between the two polynucleotide strands is correct?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Working through the options

Complementary base pairing in DNA always pairs a purine with a pyrimidine: adenine (a purine) always pairs with thymine (a pyrimidine), and guanine (a purine) always pairs with cytosine (a pyrimidine). This immediately removes any option that pairs adenine with cytosine or guanine with thymine, because those combinations never occur in a DNA double helix.

The number of hydrogen bonds is also fixed for each pair:

  • Adenine and thymine are held together by two hydrogen bonds.
  • Guanine and cytosine are held together by three hydrogen bonds (one more than A–T, which is why GC-rich DNA is slightly more thermally stable).

Checking each option against both rules (correct partner and correct bond number):

  • A. Wrong partner (adenine does not pair with cytosine).
  • B. Adenine pairs with thymine, and this pair is held by two hydrogen bonds. Correct on both counts.
  • C. Guanine does pair with cytosine, but this pair is held by three hydrogen bonds, not two, so the bond count is wrong.
  • D. Wrong partner (guanine does not pair with thymine).

Final answers

  • B, adenine pairs with thymine via two hydrogen bonds; guanine pairs with cytosine via three hydrogen bonds.