How it works
A checkpoint is a point where the cell cycle stops unless conditions are right. This is a cell cycle diagram labeled with checkpoints: the three major ones, as gates across the wheel. The second chart follows cyclin through two cycles.
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What each gate checks
The G1 checkpoint comes before S phase. A cell with damaged DNA is held there, and a cell can leave the cycle for G0. The G2 checkpoint comes before M phase and holds the cell until its DNA is copied in full and undamaged. The M checkpoint holds the cell in metaphase until spindle fibers are attached to every chromosome.
The second chart is a cell cycle regulation diagram
A cyclin-dependent kinase works only with a cyclin bound to it. The mitotic cyclin builds up during G2, peaks in M phase and is then broken down.
Questions and answers
What are the three checkpoints of the cell cycle?
The G1 checkpoint, before DNA is copied; the G2 checkpoint, before mitosis; and the M checkpoint, also called the spindle checkpoint, in metaphase. Other checks exist, but these are the three major ones.
What does a cell cycle arrest diagram show?
A cell held at a checkpoint. Arrest means the cycle stops there: at G1 or G2 when DNA is damaged, or in metaphase when a chromosome is not attached to the spindle. The cycle goes on once the fault is put right.
What is MPF?
M-phase promoting factor, also called maturation-promoting factor: the mitotic cyclin joined to its Cdk. When it becomes active the cell enters mitosis. It is switched off when the cyclin is broken down.
What is G0?
A state outside the cycle. A cell leaves from G1 and stops dividing, for a time or for good. Many G0 cells are busy doing their work in the body.