The phases of the cell cycle
Every cell comes from another cell by division. Between one division and the next, a cell goes through an ordered sequence of stages (growth, DNA duplication, checking and distribution) that is repeated in almost exactly the same way in every cell generation. This sequence is called the cell cycle, and understanding it is the basis for explaining phenomena as different as the healing of a wound or the origin of a tumour.
Interphase and M phase
If you look at a growing tissue under an optical microscope, you will see that almost all the cells show a uniform nucleus and only a few have visible chromosomes in the middle of being shared out: why do we almost never see cells dividing?
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 subphases of interphase: G1, S, G2 (and G0)
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.
Order these phases of the cell cycle as they occur over time: M phase, G1 phase, G2 phase, S phase. Briefly describe what the cell does in each.
Order these phases of the cell cycle as they occur over time: M phase, G1 phase, G2 phase, S phase. Briefly describe what the cell does in each.
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.
Why cells do not grow without limit: surface area and volume
An elephant is millions of times bigger than a mouse, but the cells of both have practically the same microscopic size: why do large organisms increase their number of cells instead of having giant cells?
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.
Control of the cycle: checkpoints
On a car assembly line, each vehicle goes through quality scanners before moving on to painting or the engine: how does a cell check that its DNA is complete and error-free before sharing it out to its daughter cells?
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:
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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.
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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 cut yourself, the skin cells receive signals that activate them to divide and repair the tissue; if, on the other hand, the checkpoints fail and the divisions do not stop when healing is finished, a tumour can arise.