Draw a labelled diagram of a mature human egg.

Outline a technique used for gene transfer.

Explain how evolution may happen in response to environmental change with evidence from examples.

Award [1] for each structure accurately drawn and correctly labelled.

a. haploid nucleus;

b. cytoplasm – with nucleus-to-membrane distance >4 times nucleus diameter;

c. centrioles – two must be shown but only one needs to be labelled;

d. cortical granules – needs to be drawn in vicinity of plasma membrane;

e. plasma membrane – shown as a single line and approximately circular overall;

f. polar cell / (first) polar body – needs to be drawn outside the egg cell;

g. zona pellucida / layer of gel (outside the cell membrane);

h. follicle cells / corona radiata (outside the cell membrane);

i. size shown as 100 μm/0.1mm; (accept 90 μm to 120 μm)

a. plasmid used for gene transfer/removed from bacteria;

b. plasmid is a small/extra circle of DNA;

c. restriction enzymes/endonucleases cut/cleave DNA (of plasmid);

d. each restriction enzyme cuts at specific base sequence/creates sticky ends;

e. same (restriction) enzyme used to cut DNA with (desired) gene;

f. DNA/gene can be added to the open plasmid/sticky ends join gene and plasmid;

g. (DNA) ligase used to splice/join together/seal nicks;

h. recombinant DNA/plasmids inserted into host cell/bacterium/yeast;

a. variation in population;

b. (variation is) due to mutation/sexual reproduction;

c. valid example of variation in a specific population;

d. more offspring are produced than can survive / populations over-populate;

e. competition / struggle for resources/survival;

f. example of competition/struggle for resources;

g. survival of fittest/best adapted (to the changed environment)/those with beneficial adaptations / converse;

h. example of changed environment and adaptation to it;

i. favourable genes/alleles passed on / best adapted reproduce (more) /converse;

j. example of reproduction of individuals better adapted to changed environment;

k. alleles for adaptations to the changed environment increase in the population;

l. example of genes/alleles for adaptations increasing in a population;

m. evolution by natural selection;

n. evolution is (cumulative) change in population/species over time / change in allele frequency;

Suitable examples are antibiotic resistance and the peppered moth but any genuine evidence-based example of adaptation to environmental change can be credited.

Of the four drawings on this exam, the egg drawings were in general the weakest. The nucleus was in almost all cases far too large and cortical granules were often distributed throughout the cytoplasm rather than being located close to the plasma membrane. Structures outside the plasma membrane were often muddled, perhaps because it was necessary to use three or more concentric circles to represent them.

Strong candidates had no difficulty in scoring full marks here by describing gene transfer using plasmids, restriction enzymes and DNA ligase. The weakest candidates wrote on a wide range of other topics.

Answers ranged from impressive, with a secure understanding of evolution by natural selection and effective use of examples, to very confused. In contrast to some previous exams most candidates chose appropriate examples such as the evolution of antibiotic resistance in bacteria or the development of melanism in peppered moths. Descriptions of the development of the giraffe’s neck or speciation in Galapagos finches were not accepted because they do not correspond with any specific environmental change for which we have good evidence. It is particularly important to base accounts of evolution on strong evidence rather than speculation, because of the objections to the theory that are still being raised.