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Date	May 2021	Marks available	4	Reference code	21M.2.HL.TZ1.8
Level	Higher level	Paper	Paper 2	Time zone	Time zone 1
Command term	Outline	Question number	8	Adapted from	N/A

Question

Cellular processes at the molecular level are regulated by enzymes.

Outline the process of DNA profiling.

[4]

Outline the role of DNA polymerase III in DNA replication.

[4]

Explain the factors that affect the rate of enzyme-controlled reactions in cells.

[7]

Markscheme

a. sample of DNA obtained from person/hair/blood/mouth/crime scene ✔

b. PCR used to amplify/make copies of DNA (in sample) ✔

c. using Taq DNA polymerase / using DNA polymerase from thermophilic bacteria ✔

d. tandem repeats amplified/used ✔

e. gel electrophoresis used to separate DNA (into bands) ✔

f. separation according to length of fragments/number of repeats
OR
fragments of same length/number of repeats travel same distance ✔

g. pattern of bands/numbers of repeats is the profile/is unique to the individual ✔

h. example of application/forensics/crime investigation/paternity ✔

Do not accept ‘determine ancestry’ for mph. Other genes/chromosomes are more often used for that.

Accept STR for (short) tandem repeat in mpd.

a. binds to template strand adjacent to a primer/at the primer ✔

b. adds nucleotides to template strand/to single stranded DNA ✔

c. using complementary base pairing ✔

d. links nucleotides with sugar-phosphate/phosphodiester bonds ✔

e. adds nucleotides/builds new strand in 5' $\to$ 3' direction ✔

f. lagging strand is built in short segments/Okazaki fragments/synthesis is discontinuous ✔

Accept A to T and G to C instead of ‘complementary’ in mpc.

For mpb it must be clear that nucleotides, not bases, are added to an existing strand of DNA. Do not accept ‘to replication fork’ for this.

a. temperature increases rate up to optimum and higher temperatures decrease rate / graph ✔

b. faster molecular movement as temperature rises (so more substrate-active site collisions) ✔

c. high temperature/heat causes denaturation/irreversible change to active site (so rate reduces) ✔

d. rate decreased if pH is above and below optimum/if pH is too high or low / graph ✔

e. pH affects shape/structure of enzyme/active site /affects ionization (of amino acids) ✔

f. increases in substrate concentration cause rate to rise towards a plateau/WTTE / graph ✔

g. greater chance of substrate-active site collisions with higher substrate concentration
OR
active sites saturated/all full at high substrate concentrations ✔

h. higher enzyme concentration increases rate (as there are more active sites) ✔

i. enzyme inhibitors/competitive inhibitors/non-competitive inhibitors reduce the rate ✔

j. end-product inhibitors switch off metabolic pathway / act on enzyme at start of pathway rate ✔
OR
allosteric site used to control enzyme activity by binding of (non-competitive) inhibitor

Graphs can be used for mpa, mpd mpf and mph but x-axis must have the variable indicated.

For mpa there must be exponential rise to optimum then faster drop.

For mpd there must be a bellshaped curve but it need not be exactly symmetrical.

For mpf and mph there must be decreasing increases in rate towards a plateau.

Examiners report

There were some good outlines of DNA profiling. Most knew at least one application of the process. There was some confusion between profiling and sequencing and the use of tandem repeats was often missed.

For the well-prepared candidate, this posed no problems and the four marks were easily earned. At the other end of the performance range almost all candidates mentioned Okazaki fragments — a benefit of giving something a distinctive name.

This was expected to be high-scoring, because it is such a standard topic, but candidates may have been running out of steam by the time they reached it and many answers were too brief, with significant points omitted.

Syllabus sections

Core » Topic 3: Genetics » 3.5 Genetic modification and biotechnology

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