How do proteins control the cell cycle and how does that control fail in cancer?
Protein control of cell division: the cytoskeleton and microtubules, the cell cycle, cyclins and cyclin-dependent kinases, cell cycle checkpoints, apoptosis, and the loss of control that leads to cancer.
An SQA Advanced Higher Biology answer on the protein control of cell division, covering the cytoskeleton and microtubules, the stages of the cell cycle, cyclins and cyclin-dependent kinases, the G1, G2 and metaphase checkpoints, apoptosis, and how the loss of control through oncogenes and tumour suppressors leads to cancer.
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What this key area is asking
The SQA wants you to explain how proteins control cell division: the role of the cytoskeleton and microtubules, the stages of the cell cycle, how cyclins and cyclin-dependent kinases drive the cycle forward, how checkpoints monitor it, the role of apoptosis, and how the loss of this control leads to cancer.
The cytoskeleton and microtubules
Drugs that disrupt microtubules (spindle poisons) stop chromosomes being separated and so block cell division, which is why some are used as cancer treatments.
The cell cycle
Cyclins and cyclin-dependent kinases
Checkpoints
Apoptosis
Apoptosis sculpts tissues during development and removes damaged or dangerous cells, balancing cell division.
Loss of control and cancer
Examples in context
Example 1. Spindle-poison chemotherapy. Some cancer drugs bind tubulin and stop microtubules forming a working spindle, so dividing cancer cells cannot separate their chromosomes and are halted at the metaphase checkpoint. This links the cytoskeleton directly to the control of division.
Example 2. p53 as the guardian of the genome. p53 is mutated in a large fraction of human cancers. Because it normally enforces the G1 checkpoint and can trigger apoptosis, losing it removes a key brake, showing how one faulty tumour suppressor undermines several layers of control.
Try this
Q1. Name the protein that microtubules are made of. [1 mark]
- Cue. Tubulin.
Q2. Explain why degrading a cyclin helps keep the cell cycle moving one way. [2 marks]
- Cue. It switches off the CDK so the cell cannot slip back, making progression through the cycle one-directional.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style4 marksExplain how cyclins and cyclin-dependent kinases control progression through the cell cycle.Show worked answer →
A 4-mark answer needs the changing cyclin level, the binding to CDK, the phosphorylation of targets and the role of degradation.
The concentration of cyclins rises and falls through the cell cycle. When a cyclin reaches a threshold concentration, it binds to and activates a specific cyclin-dependent kinase (CDK).
The active cyclin-CDK complex phosphorylates target proteins, which drives the cell from one phase of the cycle to the next.
The cyclin is then broken down, which switches off the CDK and helps make progression through the cycle one-directional.
Markers reward (1) cyclin concentration varies through the cycle, (2) cyclin binds and activates a CDK, (3) the complex phosphorylates targets to drive progression, and (4) cyclin degradation switches the CDK off.
SQA AH style3 marksDescribe the role of cell cycle checkpoints and explain the consequence of their failure.Show worked answer →
A 3-mark answer needs what checkpoints do, an example, and the consequence.
Checkpoints monitor that critical events have been completed correctly before the cell proceeds. The G1 checkpoint checks cell size, nutrients and DNA damage; the G2 checkpoint checks that DNA replication is complete and undamaged; the metaphase checkpoint checks that chromosomes are attached to the spindle.
If a checkpoint fails, a cell may divide with damaged or unreplicated DNA, or with chromosomes wrongly distributed.
This loss of control can lead to uncontrolled division and the formation of a tumour.
Markers reward (1) checkpoints monitor completion of key events, (2) a named checkpoint and what it checks, and (3) failure leads to uncontrolled division and cancer.
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Sources & how we know this
- SQA Advanced Higher Biology Course Specification — SQA (2019)