Cues like these act by changing the activity of core cell cycle regulators inside the cell. These core cell cycle regulators can cause key events, such as DNA replication or chromosome separation, to take place.
also make sure that cell cycle events take place in the right order and that one phase (such as G_1 1 start subscript, 1, end subscript) triggers the onset of the next phase (such as S).
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_1 1 start subscript, 1, end subscript cyclins, G_1 1 start subscript, 1, end subscript/S cyclins, S cyclins, and M cyclins.
each cyclin is associated with a particular phase, transition, or set of phases in the cell cycle and helps drive the events of that phase or period.
In order to drive the cell cycle forward, a cyclin must activate or inactivate many target proteins inside of the cell.
Cyclins drive the events of the cell cycle by partnering with a family of enzymes called the cyclin-dependent kinases
Cdks are kinases, enzymes that phosphorylate (attach phosphate groups to) specific target proteins
The attached phosphate group acts like a switch, making the target protein more or less active
When a cyclin attaches to a Cdk, it has two important effects
it activates the Cdk as a kinase,
but it also directs the Cdk to a specific set of target proteins, ones appropriate to the cell cycle period controlled by the cyclin.
Cdk levels remain relatively constant across the cell cycle, but Cdk activity and target proteins change as levels of the various cyclins rise and fall.
Cdks must also be phosphorylated on a particular site in order to be active
maturation-promoting factor (MPF).
when researchers found that cells in M phase contained an unknown factor that could force frog egg cells (stuck in G_2 2 start subscript, 2, end subscript phase) to enter M phase.
MPF provides a good example of how cyclins and Cdks can work together to drive a cell cycle transition.
The MPF complexes add phosphate tags to several different proteins in the nuclear envelope, resulting in its breakdown (a key event of early M phase), and also activate targets that promote chromosome condensation and other M phase events.
MPF also triggers its own destruction by activating the anaphase-promoting complex/cyclosome (APC/C), a protein complex that causes M cyclins to be destroyed starting in anaphase
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