4.6 Regulation of Cell Cycle
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Checkpoint | 检查点 | A critical control point in the cell cycle where stop and go-ahead signals can regulate the cycle. |
| Cyclin | 周期蛋白 | A cellular protein whose concentration fluctuates rhythmically; it plays an important role in regulating the cell cycle. |
| Cyclin-dependent kinase (CdK) | 周期蛋白依赖性激酶 | A protein kinase that is active only when attached to a particular cyclin. |
| Cancer | 癌症 | A disease characterized by uncontrolled cell growth and division, resulting from a failure in cell cycle regulation. |
| Apoptosis | 细胞凋亡 | Programmed cell death, often triggered if a cell fails a checkpoint due to unrepairable DNA damage. |
1. Cell Cycle Checkpoints
The cell cycle is not a continuous, unstoppable train. Instead, it is regulated by a number of internal controls known as checkpoints.
Think of checkpoints as the "traffic lights" of the cell cycle. A cell cannot proceed to the next phase until it receives specific "go-ahead" signals. These checkpoints ensure that crucial cellular processes (like growing large enough or perfectly copying DNA) are completed correctly before the cell divides.
The three major checkpoints are: * \(G_1\) Checkpoint: The most important "decision" point. If the cell receives a go-ahead signal here, it will usually complete the S, G2, and M phases and divide. If it does not, it may exit the cycle into the non-dividing \(G_0\) phase. * \(G_2\) Checkpoint: Checks if all DNA was replicated correctly during the S phase without fatal damage. * M (Spindle) Checkpoint: Occurs during metaphase of mitosis. Checks if all chromosomes are properly attached to the spindle fibers before they are pulled apart in anaphase.
2. The Molecular Regulators: Cyclins and CdKs
How does a cell actually pass a checkpoint? The rhythm of the cell cycle is driven by the rhythmic fluctuations of specific proteins. Interactions between cyclins and cyclin-dependent kinases (CdKs) control the cell cycle.
AP Exam Exclusion Statement: Specific Pairs
Knowledge of specific cyclin-CdK pairs (such as MPF, Cyclin D, Cyclin E) or specific growth factors is beyond the scope of the AP Exam. Focus strictly on the general mechanism of how they interact!
How They Interact
- CdKs (The Engine): These are enzymes (kinases) that phosphorylate other proteins to trigger cell cycle progression. They are present in the cell at a constant concentration all the time. However, on their own, they are inactive.
- Cyclins (The Key): These are regulatory proteins. As their name suggests, their concentration fluctuates cyclically—building up during interphase and rapidly degrading after mitosis.
- The Complex: When cyclin levels rise high enough, cyclins bind to the waiting CdKs. This binding activates the CdK. The active Cyclin-CdK complex then phosphorylates target proteins, acting as the "go-ahead" signal to push the cell past a checkpoint. Once the checkpoint is passed, the cyclin is degraded, turning the CdK "off" again.
3. Disruptions to the Cell Cycle
What happens when these highly regulated internal controls fail?
- Apoptosis (Programmed Cell Death): If a checkpoint detects a severe anomaly—such as massively damaged DNA or chromosomes failing to attach to the spindle—it will halt the cell cycle. If the damage cannot be repaired, the cell will initiate apoptosis. This self-destruction protects the organism from replicating damaged and potentially harmful cells.
- Cancer: Disruptions to the cell cycle can result in cancer. Cancer cells manage to escape the usual controls on the cell cycle.
- They divide excessively and invade other tissues.
- This is typically caused by genetic mutations in the proteins that regulate the cycle (e.g., a mutated CdK that is constantly active even without cyclin, or a broken checkpoint sensor that ignores DNA damage).
- Cancer cells do not heed normal "stop" signals and often evade apoptosis.
Quiz
Source: Campbell Biology Practice Test - Chapter 12 (The Cell Cycle & Regulation)