A level Biology exam revision resources written by A level Examiners


From the diagram above it can be seen that DNA is a double stranded molecule. It consists of two polynucleotide chains held together by hydrogen bonds between pairs of bases projecting towards each other from each strand.

The two polynucleotide chains in a molecule of DNA are not identical, but are complementary. The bases always pair up in a specific fashion; adenine always pairs with thymine, cytosine always pairs with guanine, i.e. a purine base pairs with a pyrimidine base.

There are several pieces of evidence that support this. Two key pieces of evidence are:

  • In any molecule of DNA, the ratio of A:T is always 1:1, and the ratio of C:G is similarly always 1:1. However, the ratio of A or T to C or G is variable between different molecules of DNA.
  • Hydrogen bonding can only occur between A and T and between C and G.

The diagram on the right shows how the double-stranded molecule is coiled up into a helical structure with ten nucleotides per turn of the helix.

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