Question

How does a cell control the cell cycle? Explain 4 different ways that cell cycle is controlled.

Answer 1

The cell cycle is carried out in two phases. They are interphase and division phase or M phase.

During interphase, there are three subphases. They are G1, S and G2 phases. Thus, the whole process is called the cell cycle and consists of four main phases.

In most of the cell cycles, the interphase accounts for 95% of the time and M phase occupies only 5% of the time of the total cell cycle.

There are several internal and external factors that decide whether the cell move forward in cell cycle from every step to its next step. These signals include certain molecular signals and the internal signal like DNA damage. If there is any such cues, they act by changing the activity of core cell cycle regulators inside the cell. These regulators cause the key events of cell cycle such as DNA replication or chromosome separation, to take place.

The most important core cell cycle regulators: proteins called cyclins, enzymes called Cdks, and an enzyme complex called the APC/C.

Cyclins are among the most important core cell cycle regulators. Cyclins are a group of related proteins, and there are four basic types found in humans and most other eukaryotes: G_11​start subscript, 1, end subscript cyclins, G_11​start subscript, 1, end subscript/S cyclins, S cyclins, and M cyclins. Each cyclin is associated with a particular phase.

Cyclins drive the events of the cell cycle by partnering with a family of enzymes called the cyclin-dependent kinases. Kinases phosphorylate specific target proteins. The attached phosphate group acts like a switch, making the target protein more or less active.

G_11​start subscript, 1, end subscript/S cyclins send Cdks to S phase targets, M cyclins send Cdks to M phase targets.

Maturation promoting factor or MPF. The MPF complexes add phosphate tags to several different proteins in the nuclear envelope, resulting in its breakdown and also activate targets that promote chromosome condensation and other M phase events.

Anaphase-promoting complex/cyclosome (APC/C) is a protein complex that causes M cyclins to be destroyed starting in anaphase. The destruction of M cyclins pushes the cell out of mitosis, allowing the new daughter cells to enter G1.

Cdks, cyclins, and the APC/C are direct regulators of cell cycle transitions, but they can't do the job all the time. So, there is one protein called p53, which acts as tumor suppressor that works on multiple levels to ensure that cells do not pass on their damaged DNA through cell division.