A DNA molecule is a double helix with two polynucleotide chains held…

The idea

Two antiparallel polynucleotide strands twist into a double helix. The sugar-phosphate backbones lie on the outside, protecting the bases, which point inwards and pair up. Hydrogen bonds hold the specific complementary pairs: adenine with thymine (two hydrogen bonds), and cytosine with guanine (three hydrogen bonds). Pairing is specific because a double-ringed purine always pairs with a single-ringed pyrimidine, keeping the backbones a constant distance apart, and only these combinations have matching hydrogen-bonding groups. Each hydrogen bond is weak, allowing strand separation for replication and transcription. However, the vast number of hydrogen bonds along the molecule makes it very stable; regions rich in cytosine and guanine are even more stable.

What earns the marks

• Two polynucleotide strands wound into a double helix.
• The strands being held together by hydrogen bonds between the bases.
• Specific complementary base pairing, adenine with thymine and cytosine with guanine.
• Two hydrogen bonds between adenine and thymine and three between cytosine and guanine.
• The sugar-phosphate backbone lying on the outside with the bases on the inside.

Common mistakes and how to put them right

• Giving base pairs as letters when the question asks you to name the bases.

  Put it right by writing the full names of the bases.

• Saying the two strands are joined by phosphodiester bonds, which join nucleotides within a strand.

  Put it right by stating that hydrogen bonds join the strands.

• Swapping the numbers, so adenine and thymine are given three hydrogen bonds and cytosine and guanine two.

  Put it right by remembering A–T has two and C–G has three.

• Describing hydrogen bonds as strong, instead of explaining that many weak bonds together give stability.

  Put it right by stating that individual hydrogen bonds are weak but numerous.

• Saying the strands are identical rather than complementary.

  Put it right by stating that the strands are complementary, with A opposite T and C opposite G.

Exam tips

• If a question mentions stability, write both halves: each hydrogen bond is weak, but there are very many of them.
• If a question mentions strand separation, the same fact works the other way: individually weak bonds are easy to break.
• State the specific base pairs clearly (adenine with thymine, cytosine with guanine) rather than just saying 'complementary base pairing', to ensure you secure the mark.