A-LevelBiologyNucleic acids and protein synthesis9700/22 · Oct/Nov 2019 · Q66 Marks

Telomerase is an enzyme that is important in maintaining the telomeres that are located at the ends of chromosomes.

(a)Explain the importance of telomeres.[2]
(b)Suggest why telomerase is present in much higher quantities in stem cells and cancer cells than in other cell types.[1]
(c)Studies suggest that telomerase is a flexible molecule when binding its substrate. The active site changes its shape slightly to allow the formation of the enzyme-substrate complex. Name the theory that describes this mode of enzyme action.[1]
(d)An investigation was carried out into the effect of substrate concentration on the activity of telomerase. The results are shown in Fig. 6.1. [Figure 6.1] With reference to Fig. 6.1, explain the difference in the rates of reaction obtained for telomerase at a substrate concentration of 0.2µmoldm⁻³ compared with a substrate concentration of 1.6µmoldm⁻³.[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[2] (a) I maintains length of chromosomes / prevents chromosome shortening any two from: permit continued replication; prevent loss of genes ; I prevents gene damage genetic material is neutral A genetic / coded, information for genes A information on DNA A protein coding regions of DNA A exons protect ends of chromosomes from being, degraded / AW ; AVP; e.g. prevents ends of chromosomes from being attached to each other not mistaken for a break in DNA that needs repairing
[1] (b) Idea that, mitosis / cell cycle / (DNA) replication / (cell) division, occurs in both cell types (continuously / regularly / AW) ; R uncontrolled mitosis R meiosis
[1] (c) induced fit;
[2] (d) allow low(er) rate for 0.2 µmol dm-³ and high(er) rate for 1.6 µmol dm-3 any two from: comparison in terms of active site(s) ; e.g. (low rate) active sites, not all occupied / spare / not all saturated or (high rate) active sites all occupied / no spare active sites / active sites saturated bod 'active sites are limited' comparison in terms of enzyme-substrate complexes formed (per unit time) e.g. (low rate) few(er) ES complexes formed or (high rate) many / more, ES complexes formed ; A (low rate) few(er) collisions between enzyme and substrate ora (low rate) substrate concentration limiting factor or (high rate) limiting factor is, enzyme concentration / not substrate concentration;

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