State whether the following is true or false.
The last doubling of a tumour adds as much mass as all previous doublings combined.
Answer: A
Each doubling adds as much as everything that came before it, which is a property of exponential growth. This is why a tumour can seem to appear suddenly after growing unnoticed for years. The early doublings involve numbers of cells far too small to detect.
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Density-dependent inhibition of growth in culture refers to cells
Answer: B
Nutrients and growth factors become limiting and cell-to-cell contact signals arrest, so growth stops at a characteristic density. Transformed cells ignore both signals and keep going. Piling up into disordered heaps is a standard sign of transformation in culture.
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Consider the following statements about cell cycle control.
1. Cyclin-dependent kinases require a cyclin partner to be active.
2. Cyclins are destroyed by the proteasome at defined points.
3. Checkpoint failure has no consequence for the daughter cells.
Answer: C
The first two are correct. The third is plainly wrong: a checkpoint exists to prevent damaged or incompletely prepared cells dividing, and its failure passes those faults to both daughters. That is precisely why checkpoint genes are so often mutated in cancer.
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The main reason a cell checks its DNA before replicating it, rather than afterwards, is that
Answer: D
Replicating a damaged template fixes the fault into both copies, so catching it beforehand is far cheaper. Repair does continue later, but by then the error may already have been propagated. Prevention is cheaper than correction here, as it usually is.
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State whether the following is true or false.
Cancer is best described as a disease of unregulated cell division.
Answer: A
The defining feature is division that continues in defiance of the normal controls, together with an ability to invade and spread. Several separate mutations are usually needed before a cell behaves that way. The requirement for several hits is why cancer risk rises steeply with age.
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Cyclin D is chiefly associated with progression through
Answer: B
Correct answer: G1 towards the restriction point
Cyclin D is the sensor of growth factor signalling: its level tracks the mitogenic environment, and with its kinase partners it phosphorylates the retinoblastoma protein, releasing E2F and committing the cell past the restriction point into S phase. Because it links external signals to that commitment, it is one of the most frequently deregulated components in cancer.
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Cyclin B is chiefly associated with
Answer: C
Together with its kinase it forms the factor that drives a cell into division, and its destruction at anaphase allows the cell to exit mitosis again. The rise and fall is one of the clearest oscillations in the cycle. Its discovery in sea urchin eggs gave the cyclins their name.
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State whether the following is true or false.
Different cyclins act at different points in the cell cycle.
Answer: D
Cyclin D operates in G1, cyclin E at the G1 to S transition, cyclin A during S and cyclin B at mitosis. Their successive appearance is what gives the cycle its ordered sequence. Substituting one for another would disrupt the order of events.
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A tissue has 20,000 cells and 500 are observed in mitosis at any moment. If the cycle lasts 24 hours, roughly how long does mitosis last?
Answer: A
The fraction in mitosis, 500 in 20,000, is one fortieth, so mitosis takes a fortieth of 24 hours, which is 36 minutes. Counting cells in a snapshot is a standard way of timing the phases. The method assumes the population is growing steadily and unsynchronised.
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Assertion (A): The proportion of cells seen in a given phase indicates how long that phase lasts.
Reason (R): In an unsynchronised population, cells are distributed across the cycle in proportion to phase length.
Answer: B
Both are true and the reason supplies the logic: a phase occupying a tenth of the cycle will contain about a tenth of the cells at any instant, which is what makes a single fixed sample informative. No time-lapse observation is needed to estimate the phase durations.
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