The cell cycle does not take the same amount of time in every cell. Its timing depends on the cell type, developmental stage, and signals inside and outside the cell. This lesson introduces cell-cycle variation, external signals that affect division, and the G1 checkpoint that determines whether a cell can begin copying its DNA.
Some cells divide rapidly, while others divide rarely or not at all. In humans, cell turnover can take only a few hours during early embryonic development. Many epithelial cells are replaced every two to five days. Specialized cells such as cortical neurons and cardiac muscle cells may remain in the nondividing state called G0 for an entire lifetime.
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What is G0?
G0 is a nondividing state in which a cell has left the active cell cycle. Some specialized cells may remain in G0 for a very long time, even for their entire lifetime.
What is a cell-cycle checkpoint?
A checkpoint is a control point where cell-cycle progression can be halted until conditions are suitable for moving to the next stage.
What does the G1 checkpoint assess?
It assesses conditions such as cell size, available resources and protein reserves, external growth signals, and whether genomic DNA is damaged.
Why can cell growth encourage division?
As a cell grows, its surface-to-volume ratio decreases. Division produces smaller cells that can exchange materials more efficiently relative to their volume.
What can a cell do if it fails the G1 checkpoint?
It can pause and attempt to correct the problem, or enter G0 and wait for conditions or signals to improve instead of proceeding into S phase.
Quiz questions
How often are many human epithelial cells replaced?Answer: On average, every two to five days.
Why can early fruit-fly embryo cell cycles be completed in about eight minutes?Answer: Nuclei divide repeatedly, while cytokinesis is delayed until many nuclei have formed.
What does the G2 checkpoint verify before a cell enters mitosis?Answer: That DNA replication is complete and the replicated DNA is not damaged.
What must be in place for a cell to pass the M checkpoint and begin anaphase?Answer: Sister chromatids must be attached to spindle fibers from opposite poles.
How do cyclin and Cdk protein levels typically differ during the cell cycle?Answer: Cyclin levels fluctuate, while Cdk levels remain relatively stable.
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.