Draw a labelled diagram showing the ultra-structure of a liver cell.

Distinguish between prokaryotic cells and eukaryotic cells.

Explain prokaryotic DNA replication.

Award [1] for each structure clearly drawn and correctly labelled. Whole cells not necessary.
(plasma) membrane – single line surrounding cytoplasm;
nucleus – with a double membrane and pore(s) shown;
mitochondria(ion) – with a double membrane, the inner one folded into internal projections, shown no larger than half the nucleus;
rough endoplasmic reticulum – multi-folded membrane with dots/small circles on surface;
Golgi apparatus – shown as a series of enclosed sacs with evidence of vesicle formation;
ribosomes – dots/small circles in cytoplasm/ribosomes on rER;
lysosome;
Award [0] if plant cell is drawn. Award [2 max] if any plant cell structure (e.g. cell wall) is present.

DNA replication is semi-conservative / each strand of DNA acts as template;
(DNA) helicase separates two strands/forms a replication fork;
new strand built / nucleotides added in a 5' to 3' direction;
(deoxy)nucleoside triphosphates hydrolysed to provide energy for nucleotide formation/base pairing;
on one strand DNA polymerase III builds continuous strand;
on other strand short chains of DNA/Okazaki fragments are formed;
each short chain starts with RNA primer;
added by RNA primase;
then remainder of chain of DNA built by DNA polymerase III;
DNA polymerase I removes RNA primer and replaces it by DNA;
DNA ligase joins DNA fragments together forming complete strand;
replication only occurs at a single replication fork;
Award credit for any of the above points clearly drawn and accurately labelled.

In the light of answers seen by examiners, perhaps the question should have given candidates a clearer pointer to what was expected. The quality of drawings was very variable. Marks were only awarded for structures clearly drawn and labelled. The mark scheme for this paper gives details of the criteria that examiners used. It was not necessary to draw a whole cell, as this would have involved drawing organelles repeatedly, but at least one of each organelle type, accurately drawn, was needed.

This was often answered by means of a table. This was particularly appropriate here as the question asked for prokaryote and eukaryote cell structure to be distinguished, rather than compared, so only differences were required. Tables help to ensure that candidates give both sides of a distinguishing feature. This approach only works if candidates fully understand the features, which they did not in some cases. For example, naked DNA in prokaryotes was often matched with DNA enclosed in a nucleus in eukaryotes, rather than with DNA associated with histone proteins. Mesosomes were given as an equivalent of mitochondria although most bacteriologists now regard the mesosome as an artefact of preparation for electron microscopy, rather than as a functionally significant structure. The current IB Biology programme does not refer to mesosomes.

This may also have discouraged answers from some candidates, as it referred to DNA replication in prokaryotes. This is how assessment statement 7.2.2 is phrased, so the wording of the question was acceptable, but there were some answers that showed some candidates had been confused. Some wrote about binary fission, about the replication of a circular DNA molecule, or even about the cell cycle and mitosis. However, stronger candidates coped extremely well and quickly amassed eight marks. The best answers explained the method of replication on the leading strand and then explained how and why the process was different on the lagging strand.