Draw a labelled diagram showing two different complementary pairs of nucleotides in a molecule of DNA.

Outline the structure of nucleosomes.

Explain primary structures and tertiary structures of an enzyme.

The structures underlined must be labelled.
at least one nucleotide with deoxyribose linked to base and phosphate;{ Labels need not be on the same nucleotide. Do not allow sugar.
phosphate and deoxyribose linked C₃ to C₅;{ Position required, not label. Straight line from C₄ to phosphate is acceptable. Do not penalize if the second strand is not antiparallel and the bonding is therefore incorrect on it.
(complementary) bases labelled with at least one of each of A, G, T and C correctly linked to C₁;
hydrogen bonds between correct complementary bases;{ Bond numbers not required.
correct antiparallel orientation shown; (as seen by shape or orientation of sugar)

(eight) histone (proteins);
DNA wrapped around histones/nucleosome;
further histone holding these together;
Do not allow histone wrapped around DNA.

primary structure is (number and) sequence of amino acids;
joined by peptide bonds;
tertiary structure is the folding of the polypeptide/secondary structure/alpha helix;
stabilized by disulfide/ionic/hydrogen bonds/hydrophobic interactions;
tertiary structure gives three dimensional globular shape/shape of active site;

This was often well answered, with many candidates scoring four marks. The sugar was sometimes labelled as ribose rather than deoxyribose, or simply as sugar. Another common error was to link the phosphate groups to the oxygen in the sugar ring, rather than to C₄ via C₅.

Stronger candidates often drew impressively detailed and accurate diagrams, with the antiparallel orientation of the strands, the numbers of hydrogen bonds and the molecular structure of deoxyribose and phosphate groups correctly shown. It was possible to score four marks without all of this detail, but it was good to see such high quality answers.

This was also well answered by properly prepared candidates. A few misread the question and outlined the structure of nucleotides rather than nucleosomes.

This was more poorly answered than expected. Perhaps candidates who knew about the primary and tertiary structure of proteins were unable to transfer this knowledge to a question about enzymes, though they surely knew that enzymes are globular proteins. Many of the candidates who did write about primary and tertiary structure failed to include the essential detail that primary structure is the sequence or order of amino acids. There was some confusion between secondary and tertiary structure and also some over-simplified accounts of tertiary structure. Some candidates stated simply that tertiary structure is three-dimensional structure. It was expected that candidates should at least include the idea that enzymes are globular in their three-dimensional structure.