Outline, using graphs, the effect of different factors that influence enzyme activity.

Describe the function of three named enzymes involved in DNA replication.

Explain how speciation occurs, including the different processes of isolation and selection.

1. enzymes have active sites that bind specific substrates;
2. act as catalysts to speed up reactions
   OR
   lower activation energy;
3. rate/activity increases with temperature;
4. up to an optimum temperature;
5. sharp decline in activity above (optimum temperature);
   (Graph has to be clearly asymmetrical for mpe)
   (Graph shown would earn mpc, mpd and mpe)
   ;
6. rate/activity declines at a pH above and below the optimum pH
   (Graph shown would earn mpf)
   ;
7. rate/activity increases sharply as substrate concentration goes up;
8. above a certain concentration, the rate reaches a maximum/plateau;
   (Graph shown would earn mpg and mph)
   ; 
9. competitive inhibitor present, as substrate concentration increases, enzyme requires higher concentration to achieve maximum rate / graph showing this with and without inhibitor;
10. non-competitive inhibitor present, as substrate concentration rises, enzyme activity is lower at all substrate concentrations / graph showing shape with and without inhibitor;

Award [3 max] if there are no graphs.

For each graph, axes must be correctly labelled, the shape must be correctly drawn.

Marks can be awarded to a correctly annotated graph.

1. Helicase separates/unwinds DNA from double helix;
2. (DNA) gyrase / topoisomerase releases tension/strain (caused by super coiling);
3. (DNA) primase builds/forms/adds an RNA primer;
4. DNA polymerase I is a (5’ $\to$ 3’) exonuclease/removes RNA primers/replacement of RNA by DNA;
5. DNA polymerase III synthesizes DNA (5’ $\to$ 3’) on leading/lagging strands/forms bonds between DNA nucleotides;
6. (DNA) ligase connects/seals nick between Okazaki fragments to make continuous DNA strand;

Only mark the first three answers in sequence.

DNA polymerase I required for mpd and DNA polymerase III required for mpe (i.e., the numbers are required).

1. Species is a group of organisms that interbreeds (normally in the wild) and produce fertile offspring;
2. within an interbreeding population there is variety / variation exists;
3. some adaptations favour survival to reproductive age /survival of the fittest / natural selection;
4. alleles for these adaptations become more frequent/are inherited in the population /change with time;
5. speciation is the formation of new species;
6. (speciation) occurs because populations have become reproductively isolated / no longer able to interbreed / exposed to different selection pressures;
7. behavioural isolation involves differences in courtship or mating behaviours;
8. temporal isolation involves differences in the timing of courtship or mating behaviours;
9. geographical isolation / allopatric refers to the physical barriers that exist that keep two populations from mating;
10. polyploidy can lead to reproductive isolation;
11. stabilizing selection is when the two extremes of a trait have lower reproductive fitness (OWTTE) / favours average phenotype;
12. directional selection is when one extreme of the trait has lower reproductive fitness (OWTTE);
13. disruptive/diversifying selection favours both extreme phenotypes / intermediate phenotype has lower fitness;

This question was the most popular choice of essay as well as the most successful.

Many earned full marks, but many others contained errors. Some of these errors included inverted axes, no abrupt curve drop for high temperatures, an asymmetric graph shown for pH, the optimum temperature or pH not mentioned or labelled.

This question about the enzymes involved in replication was generally well done. Some omitted the I and the III for polymerase and/or focused on base pairing instead of elongation of strands with DNA polymerase III. The details around the relative role of base pairing and covalent bond formation was sometimes lacking.

Many had a "good idea" of speciation and knew about the different types of isolation and selection. Very few discussed the role of polyploidy.