Maturation promoting factor, which drives a cell into mitosis, is a complex of
Answer: A
Its discovery in frog eggs showed that a diffusible factor could push a cell into division, and it was later identified as a cyclin-CDK pair. The finding linked embryology to yeast genetics. Two quite separate research traditions turned out to be studying one system.
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Yeast has been especially valuable for studying the cell cycle because it
Answer: B
Temperature-sensitive mutants that arrest at particular points allowed the genes to be identified one by one. Many of those genes turned out to have close human counterparts. Work in yeast therefore illuminated human cancer directly.
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State whether the following is true or false.
Genes controlling the cell cycle are broadly conserved between yeast and humans.
Answer: C
Correct answer: True
The conservation is close enough that human cyclin-dependent kinase genes can rescue yeast mutants lacking their own. That is how the machinery was identified: the cdc mutants of budding and fission yeast defined the components, and the human counterparts were then found by complementation - work that earned the 2001 Nobel Prize and that underlies the cell cycle drugs now used in oncology.
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A cell cycle of 20 hours has G1 of 9 hours, S of 6 hours and G2 of 4 hours. How long does mitosis take?
Answer: D
Adding the three named phases gives 19 hours, leaving one hour for mitosis. The proportions are typical, with the visible part of the cycle being much the shortest. Interphase dominates the cycle in almost every cell type.
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A cell arrested at the G2 checkpoint has most likely failed to
Answer: A
The G2 checkpoint tests whether replication finished and whether the DNA is intact before mitosis begins. Size is assessed at G1 and spindle attachment during mitosis itself. Each checkpoint guards the step immediately after it.
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Assertion (A): A cell with unreplicated DNA does not normally enter mitosis.
Reason (R): The G2 checkpoint detects incomplete replication and blocks progression.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason names the mechanism. The G2 checkpoint monitors whether replication is complete and whether the DNA is damaged, and it holds the cyclin-dependent kinase driving mitotic entry inactive until both are satisfied. Entering mitosis with unreplicated DNA would tear the chromosomes apart at anaphase, so the block is essential rather than merely prudent.
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Many chemotherapy drugs act preferentially on cancer cells because those cells
Answer: C
Agents that damage DNA or disrupt the spindle harm dividing cells most, which is why hair follicles and the gut lining also suffer. The selectivity is a matter of degree rather than of kind. Newer targeted therapies aim to exploit differences that are more specific.
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State whether the following is true or false.
Chemotherapy side effects such as hair loss reflect damage to normal dividing tissues.
Answer: D
Correct answer: True
Most cytotoxic agents act on dividing cells, and they cannot distinguish a tumour cell from a normal one that happens to be dividing. The tissues that suffer are therefore those with the highest turnover - hair follicles, the lining of the gut, and the bone marrow - which is why alopecia, mucositis and myelosuppression are the characteristic side effects, and why they recover once treatment stops.
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Consider the following statements about cell cycle checkpoints.
1. The G1 checkpoint assesses DNA damage and cell size.
2. The spindle checkpoint acts during mitosis.
3. Checkpoints operate only in cancer cells.
Answer: A
The first two are correct. The third inverts the truth, since checkpoints are a normal feature that cancer cells commonly lose, and it is that loss which allows damaged cells to keep dividing. Checkpoints are a safeguard that cancer characteristically dismantles.
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Apoptosis is triggered rather than repair attempted when
Answer: B
Removing a badly damaged cell protects the organism from the mutations it would otherwise pass on. p53 makes this decision, arresting the cycle first and choosing death only if repair fails. The choice between arrest and death is one of the most consequential a cell makes.
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