A-LevelBiologyThe mitotic cell cycle9700/23 · May/June 2024 · Q510 Marks

Cyclin-dependent kinases (CDKs) are enzymes that regulate the cell cycle.

(a)Cell signalling by ligands causes the activation of CDKs in target cells. Outline the main stages in the process of cell signalling by ligands that can cause specific responses in target cells.[2]
(b)The activity of CDKs is reduced by CDK inhibitors. Many of these inhibitors occur naturally in cells. Fig. 5.1 is a diagram of a CDK inhibitor binding to a CDK molecule. [Figure 5.1] State and explain how the CDK inhibitor in Fig. 5.1 prevents the activity of the CDK molecule.[2]
(c)Table 5.1 lists three different CDKs, their roles in the cell cycle and molecules that inhibit them. [Table 5.1]
(c)(i)With reference to Table 5.1, state which CDK inhibitor is likely to result in a cell containing one chromatid per chromosome. Explain your answer.[2]
(c)(ii)With reference to Table 5.1, state which CDK inhibitor is likely to result in a cell with: • a relatively high concentration of mitochondria • two chromatids per chromosome. Explain your answer.[2]
(d)Scientists have developed CDK inhibitors that are synthetic. Explain why CDK inhibitors can be used to treat cancerous tumours.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

[2] any two from:
cells secrete ligands or transport of ligands, through blood / in circulatory system ;
ligands bind to (complementary / specific) receptors ; context of target cells
(binding) sets off reactions within the (target) cell / triggers secondary messengers/sets off an enzyme cascade ; A cyclic AMP for secondary messengers
[2] (non-competitive inhibitor) binds to, allosteric site / site other than active site / AW
(binding causes) change in shape of active site ;
active site no longer complementary to the substrate / AW ;
[2] Palbociclib/p21Cip1;
(in S phase), stops DNA replication occurring / DNA replication occurs or identical /sister, chromatid formed in S phase ; A idea of p21Cip1 disrupts normal DNA replication (in S phase)
[2] RO-3306, qualified
1 cell cycle stops (in G2) after mitochondria have, divided / increased in number or mitochondria, divide / increase in number, in (G1 and) G2 / before mitosis; A no, mitosis / cytokinesis, so numbers of mitochondria remain high
2 idea that DNA replication will have occurred / sister chromatids will have formed, in S phase, but the cell cannot carry out mitosis;
[2] cell cycle is stopped before, mitosis / cytokinesis / cell division ;
idea of stops uncontrolled cell division (increasing tumour size);

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