Distinguish between RNA and DNA.

Explain the process of DNA replication.

Outline how enzymes catalyse reactions.

DNA is double-stranded while RNA is single-stranded;
DNA contains deoxyribose while RNA contains ribose;
the base thymine found in DNA is replaced by uracil in RNA;
one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);

occurs during (S phase of) interphase/in preparation for mitosis/cell division;
DNA replication is semi-conservative;
unwinding of double helix / separation of strands by helicase (at replication origin);
hydrogen bonds between two strands are broken;
each strand of parent DNA used as template for synthesis;
synthesis continuous on leading strand but not continuous on lagging strand;
leading to formation of Okazaki fragments (on lagging strand);
synthesis occurs in 5'→3' direction;
RNA primer synthesized on parent DNA using RNA primase;
DNA polymerase III adds the nucleotides (to the 3' end)
added according to complementary base pairing;
adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone.)
DNA polymerase I removes the RNA primers and replaces them with DNA;
DNA ligase joins Okazaki fragments;
as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates broken off releasing energy to form bond;
Accept any of the points above shown on an annotated diagram.

they increase rate of (chemical) reaction;
remains unused/unchanged at the end of the reaction;
lower activation energy;
activation energy is energy needed to overcome energy barrier that prevents reaction;
annotated graph showing reaction with and without enzyme;
substrate joins with enzyme at active site;
to form enzyme-substrate complex;
active site/enzyme (usually) specific for a particular substrate;
enzyme binding with substrate brings reactants closer together to facilitate chemical reactions (such as electron transfer);
induced fit model / change in enzyme conformation (when enzyme-substrate/ES complex forms);
making the substrate more reactive;

Many of the candidates scored full marks.

Despite some confusion about which enzyme does what and confusing DNA replication with transcription/translation, many candidates managed to gain full marks. A good number indicate that an RNA primer begins replication on the lagging strand only. Another common error was to refer to the gaps rather than the fragments as Okazaki fragments. Some candidates confused replication with translation.

Many candidates lost marks by focusing on factors affecting the rate of enzyme controlled reactions and inhibition and missed the basics. Nearly all mentioned the lowering of activation energy, but many were not able to describe how this is done. Diagrams that were included could have earned more marks if they were more carefully drawn, with axes labels being more carefully included and differences in energy between reactants and products being more accurately represented. Few indicated that the enzyme was not used up in the reaction.