Regulation of the Cell Cycle

Biology: IB Diploma Higher Level, topic 2.4.3

Each step of the cell cycle is closely monitored by external signals and internal controls called checkpoints.

Internal checkpoints are important in the regulation of the cell cycle.
Regulation of the cell cycle is particularly important as disruptions can lead to tumor development or apoptosis (programmed cell death).
There are three major checkpoints in the cell cycle: one near the end of G1, a second at the G2/M transition, and the third during metaphase.

At the first checkpoint (G1), DNA is checked to make sure there is no damage and the cell is checked to see if it is the correct size and has enough energy reserves.
At the second checkpoint (G2), chromosomes are checked to make sure that they have all been replicated and none of the duplicated DNA is faulty.
At the third checkpoint (M), sister chromatids are checked to confirm that their attachments to the spindle are suitable.

Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are responsible for the progress of the cell through the various checkpoints.
The levels of the four cyclin proteins fluctuate throughout the cell cycle in a predictable pattern.
Increases in the concentration of cyclin proteins are triggered by both external and internal signals.
After the cell moves to the next stage of the cell cycle, the cyclins that were active in the previous stage are degraded.

Cyclins regulate the cell cycle only when they are tightly bound to Cdks.
To be fully active, the Cdk/cyclin complex must also be phosphorylated in specific locations.
Phosphorylation activates the protein by changing its shape.
The proteins phosphorylated by Cdks are involved in advancing the cell to the next phase.
Different cyclins and Cdks bind at specific points in the cell cycle and thus regulate different checkpoints.

Test yourself on Regulation of the Cell Cycle

How many checkpoints are involved in the regulation of the cell cycle?
Cyclins regulate the cell cycle only when they are tightly bound to which type of protein?