Syklisiin

Syklisiin is a term used in Finnish-language biology texts to refer to cyclins, a family of regulatory proteins that control the progression of the eukaryotic cell cycle by activating cyclin-dependent kinases (CDKs). Cyclins do not have catalytic activity themselves; their primary role is to bind and activate CDKs, thereby determining the timing and substrate specificity of cell cycle events. Their expression rises and falls in a cyclical pattern that matches the cell’s progression through G1, S, G2, and M phases.

In many organisms, cyclins are grouped by the phase during which they peak. Common examples include D-type cyclins (G1), E-type cyclins (G1/S), A-type cyclins (S and G2), and B-type cyclins (M). Notable human cyclins include cyclin D1, cyclin E, cyclin A, and cyclin B. The active cyclin–CDK complexes phosphorylate a range of targets to drive cell-cycle transitions, promote DNA replication, and ensure proper mitotic progression. The levels of cyclins are tightly controlled by the ubiquitin–proteasome system, leading to their degradation at specific points in the cycle, which helps reset the cell-cycle machinery for the next round.

Regulation of cyclins involves transcriptional control by growth signals, post-translational regulation, and inhibitors that restrain CDK activity. Abnormal cyclin expression or CDK activity is common in cancer and other diseases; for example, overexpression of cyclin D1 or cyclin E can contribute to uncontrolled cell division. Therapeutically, CDK inhibitors are used to target cyclin–CDK activity in various cancers.

Historically, the discovery of cyclins in the 1980s highlighted their cyclical nature, a finding that has since been confirmed across a wide range of eukaryotes, from yeast to humans.