There are four types of cycles whose concentration determines the activation of a CDK and the pass of a control.

Cyclins were originally named because their concentration varies cyclically during the cell cycle. (Note that the cyclins are now classified according to their conserved cyclin box structure, and not all these cyclins alter in level through the cell cycle.)

The oscillations of the cyclins, namely fluctuations in cyclin gene expression and destruction by the ubiquitin-mediated proteasome pathway, induce oscillations in CDK activity to drive the cell cycle.

A cyclin forms a complex with CDK, which begins to activate but the complete activation requires phosphorylation, as well. Complex formation results in activation of the CDKactive site.

Cyclins themselves have no enzymatic activity but have binding sites for some substrates and target the Cdks to specific subcellular locations.

Cyclins, when bound with the dependent kinases, such as the p34/CDC2/CDK1 protein, form the maturation-promoting factor. MPFs activate other proteins through phosphorylation. These phosphorylated proteins, in turn, are responsible for specific events during cycle division such as microtubule formation and chromatin remodeling.

Cyclins can be divided into four classes based on their behavior in the cell cycle of vertebrate somatic cells and yeast cells: G1 cyclins, G1/S cyclins, S cyclins, and M cyclins. This division is useful when talking about most cell cycles, but it is not universal as some cyclins have different functions or timing in different cell types.

 Cyclines G1 / S  It activates CDK at the end of G1 and allows entry into phase S.

S cyclines

Stimulates DNA duplication and controls some elements of mitosis.


  • What is mitosis?

The primary mechanism by which organisms generate new cells is through cell division. During this process, a single "parent" cell will divide and produce identical "daughter" cells. In this way, the parent cell passes on its genetic material to each of its daughter cells. First, however, the cells must duplicate their DNA. Mitosis is the process by which a cell segregates its duplicated DNA, ultimately dividing its nucleus into two. 

Cyclines M

They stimulate entry into mitosis. 

It also controls inhibition.

Phosphorylation / dephosphorylation of CDK       

Importance of check (control) points

Make sure that the cycle processes are carried out correctly: cells with damaged genetic material can become cancerous. 

The cycle can be stopped: if there is damage to the DNA and if there are processes such as cell duplication or chromosome ringing that are not carried out correctly.

In these cases, repair mechanisms (polymerases, ligases ...) cause irreparable damage: cell death and conversion into a permanent stop (senescence). Ex: UV damage.

ssDNA is produced: an ATR kinase is activated that stops the cycle in the S2 state. Ex: DNA breakage damage (ionizing radiation) ends cyclin CDK inhibition.