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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The proteasome contributes to cell cycle control by
Answer: C
Abrupt destruction of cyclins and of the proteins holding sister chromatids together makes each transition sharp and irreversible. Regulation by degradation is what gives the cycle its one-way character. Synthesis and destruction together produce the oscillation.
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State whether the following is true or false.
Cell cycle transitions are made irreversible chiefly by destroying key regulatory proteins.
Answer: D
Correct answer: True
Irreversibility is achieved by proteolysis. The anaphase promoting complex and the SCF complex tag key regulators with ubiquitin and the proteasome destroys them, and because a destroyed protein cannot be restored by reversing a reaction, the transition cannot run backwards. Phosphorylation alone would be reversible by a phosphatase; degradation is what makes the ratchet.
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Separase, activated at the metaphase to anaphase transition, acts by
Answer: A
Correct answer: cutting the cohesin holding sister chromatids
Separase is a protease, and its substrate is the cohesin ring encircling the sister chromatids. Cleaving it releases the sisters so the spindle can draw them apart, which is why anaphase begins abruptly and all together rather than chromosome by chromosome. Until that moment separase is held inactive by securin, whose destruction is the trigger.
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Assertion (A): Sister chromatids remain joined until every one of them is attached to the spindle.
Reason (R): The spindle checkpoint keeps separase inhibited while any kinetochore is unattached.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. An unattached kinetochore generates a signal that inhibits the anaphase promoting complex, so securin survives, separase stays blocked and cohesin holds. Only when the last kinetochore is attached does the signal cease and the cascade run. That is why a single lagging chromosome delays the whole cell, and why the checkpoint guards against aneuploidy.
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A cell exposed to ionising radiation during G1 will most likely
Answer: C
Damage is detected and p53 halts the cycle so that repair can be attempted. Whether the cell resumes or dies depends on how far the repair succeeds. Radiotherapy relies on tumour cells handling this damage worse than normal ones.
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If a tumour of 1 gram doubles every 30 days, roughly how long until it reaches about 1 kilogram?
Answer: D
Reaching a thousand times the starting mass takes about ten doublings, since two to the tenth is 1,024, and ten periods of 30 days is 300. The last doubling alone adds half the final mass. Exponential growth means most of the mass appears in the final few doublings.
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