When you get a scrape playing baseball or volleyball, within a few days the skin repairs itself. The nearby cells divide to fill the gap, but they always stop in time. How does a cell know when to divide and when to stop? To answer that, you need to know about the cell cycle: the ordered sequence of stages (grow, copy the DNA, check, and distribute) that repeats from one cell to its daughter cells.
Interphase and M Phase
If you look at a growing tissue under the microscope, like the tip of an onion root, you will see that almost all the cells have a uniform nucleus and only a few show chromosomes in the middle of being distributed. Why do we almost never see cells dividing?
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Explain what the cell cycle is and name its two main periods, stating what generally happens in each.
Explain what the cell cycle is and name its two main periods, stating what generally happens in each.
The diagram represents the cell cycle as a circle divided into four numbered stages. Name stages 1, 2, 3 and 4, and explain why the cycle is represented as a circle…
The diagram represents the cell cycle as a circle divided into four numbered stages. Name stages 1, 2, 3 and 4, and explain why the cycle is represented as a circle rather than a line with a beginning and an end.
In a cell culture, the complete cell cycle lasts $24\ \text{h}$ and interphase takes up $90\ \%$ of the time. Calculate how many hours each cell spends in interphase and…
In a cell culture, the complete cell cycle lasts $24\ \text{h}$ and interphase takes up $90\ \%$ of the time. Calculate how many hours each cell spends in interphase and how many in the division phase.
The complete order of the cycle is: G1 → S → G2 → M phase, and then back to G1 in each daughter cell. That is why it is drawn as a circle: it has no beginning or end, since each daughter cell starts another cycle.
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What fundamental event occurs during the S phase of the cell cycle? Explain why it is essential for it to occur before the cell divides.
What fundamental event occurs during the S phase of the cell cycle? Explain why it is essential for it to occur before the cell divides.
A cell in G1 phase has a certain amount of DNA. State at what point in the cycle this amount has doubled and at what point it is halved again in each daughter cell.
A cell in G1 phase has a certain amount of DNA. State at what point in the cycle this amount has doubled and at what point it is halved again in each daughter cell.
An elephant is much bigger than a mouse, but their cells are almost the same microscopic size. Why do large animals have more cells instead of giant cells?
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Consider two spherical model cells: cell A (radius = 1 µm) and cell B (radius = 3 µm). a) Knowing that the surface area of a sphere is $S = 4\pi r^2$ and its volume is…
Consider two spherical model cells: cell A (radius = 1 µm) and cell B (radius = 3 µm).
a) Knowing that the surface area of a sphere is $S = 4\pi r^2$ and its volume is $V = \frac{4}{3}\pi r^3$, calculate the surface-to-volume ratio ($S/V$) for each of them.
b) The exchange of gases and nutrients with the environment depends on membrane surface area, while metabolic demand depends on cytoplasmic volume. Based on your numerical calculations, justify why cell B is at a critical limit of viability if it does not divide.
c) Certain highly active human cells, such as the enterocytes of the intestinal epithelium, reach a considerable size without dividing. Deduce what morphological adaptation in their membrane allows them to maintain a high $S/V$ ratio.
In a factory, each product goes through a quality check before moving on to the next step. How does a cell check that its DNA is complete and undamaged before distributing it to its daughter cells?
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In an oncology laboratory, two tumour cell cultures are analysed: Line A: has an inactivated p53 gene, so the cells pass the $G_1/S$ checkpoint even if their DNA shows…
In an oncology laboratory, two tumour cell cultures are analysed:
Line A: has an inactivated p53 gene, so the cells pass the $G_1/S$ checkpoint even if their DNA shows breaks and damage caused by ultraviolet radiation.
Line B: has an inactivated metaphase checkpoint (spindle assembly checkpoint), so the cells enter anaphase even if there are chromosomes that have not properly attached to the microtubules.
a) Predict which type of genetic alteration will predominate in the daughter cells of each culture after several divisions: point mutations at the DNA sequence level or karyotypic alterations in chromosome number (aneuploidies)? Justify your deduction in each case.
b) An antitumour drug that destabilises spindle microtubules is tested. Justify with reasoning in which of the two cell lines ($A$ or $B$) this drug will be unable to halt cell cycle progression.
After a stroke (cerebrovascular accident), destroyed neurons do not regenerate and permanent neurological sequelae usually remain. In contrast, after a superficial burn…
After a stroke (cerebrovascular accident), destroyed neurons do not regenerate and permanent neurological sequelae usually remain. In contrast, after a superficial burn that destroys thousands of epidermal cells, the skin is capable of regenerating completely within a few days.
a) Justify this medical difference by explaining the cell cycle state of the mature cells in both tissues.
b) A research team proposes reactivating the cell cycle in adult neurons to repair brain injuries by forcing them out of the G0 phase. Predict what difficulties or functional risks forcing mitosis could entail in highly specialized cells integrated into stable synaptic circuits.
Back to the beginning: when you get a scrape, the skin cells receive signals that make them divide and repair the tissue, and the checkpoints stop them when they are done. If those controls fail, the divisions do not stop and a tumor can form.
Summary
Let's practice! 3 medium exercises in this section