cell cycle
Meaning of cell cycle: Most cells in the body are not in the cycle at all, so the interesting question is what pulls them back into it.
Definition of cell cycle
The cell cycle is the ordered sequence a cell passes through in order to divide into two. It runs through a growth phase, a phase copying the entire set of chromosomes, a second growth phase preparing for division, and division itself. Checkpoints between the phases hold the cell back until the previous step has been completed correctly.
What happens in each phase?
| Phase | What happens |
|---|---|
| G1 | The cell grows and decides whether to commit to dividing |
| S | The whole set of chromosomes is copied, giving two of each |
| G2 | Growth continues while the copies are checked for damage |
| M | The chromosomes are separated and the cell splits in two |
| G0 | A resting state outside the cycle, occupied by most mature cells |
G1, S and G2 together are called interphase, which under a microscope looks like a cell doing nothing even though nearly all the work happens there. Only the M phase is visible as an event, which is why the older name mitotic cycle names the part that can be watched. G0 is not a pause inside the cycle but a state outside it, and which cells can leave it decides which tissues repair themselves. Nerve cells and heart muscle enter G0 and effectively never return; liver cells sit there for years and re-enter when the organ is injured; the cells lining the gut barely stop at all.
What do the checkpoints do?
Each checkpoint halts the cycle unless conditions are met. The one at the end of G1 asks whether the cell is large enough, whether growth signals are present and whether the DNA is intact, and passing it commits the cell to a complete round of division. The one at the end of G2 confirms that copying finished and damage was repaired. The one during division confirms that every chromosome is properly attached before the pairs are pulled apart, which is what keeps the count correct in the two daughter cells. Two proteins dominate these controls: the retinoblastoma protein holds a cell in G1 until growth signals release it, and p53 stops the cycle after DNA damage and, when repair is impossible, drives the cell to destroy itself. Faults in these two are among the most frequent genetic changes found in cancer, precisely because losing them lets a damaged cell keep dividing.
Why does it matter in cancer treatment?
Many cancer drugs act only at a particular point in the cycle, so they hit fast-dividing populations hardest and bone marrow, hair follicles and the gut lining suffer alongside the tumor. Antimetabolites interfere with copying and therefore act in S phase; drugs targeting the spindle act in M phase; alkylating agents damage DNA whenever they encounter it and are not tied to any phase. Cells resting in G0 are largely untouched by phase-specific drugs, one reason a slow-growing tumor responds poorly to treatment that works well against a fast one. A newer approach blocks the enzymes releasing the G1 checkpoint, holding cancer cells out of the cycle rather than poisoning them inside it.
Also known as
Used in a sentence
Laboratory studies examined how the drug affected the tumor cells at each stage of the cell cycle.
Good to know
Many cancer treatments act on cells only during a specific stage of the cell cycle, which is why they affect rapidly dividing cells more than slower-growing ones and why cells resting outside the cycle are largely spared.