Cyclins are a family of proteins involved in the progression of cells through the cell cycle. A cyclin forms a complex with the cyclin-dependent kinase (Cdk), which activates the latter's protein kinase function. Cyclins are so named because their concentration varies in a cyclical fashion during the cell cycle; they are produced or degraded as needed in order to drive the cell through the different stages of the cell cycle. When its concentrations in the cell are low, cyclin detaches from Cdk, inhibiting the enzyme's activity, probably by causing a protein chain to block the enzymatic site (1,2). A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... The cell cycle, or cell division cycle, is the cycle of events in a eukaryotic cell from one cell division to the next. ... Cyclin-dependent kinase is a protein kinase involved in regulation of the cell cycle. ... A protein kinase is an enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation). ...

There are several different cyclins which are active in different parts of the cell cycle and which cause the Cdk to phosphorylate different substrates. However, there are several "orphan" cyclins which no Cdk partner has been identified. For example, cyclin F is one of orphan cyclin that is essential for G2/M transition (3,4).

One major cyclin is cyclin B, a mitotic cyclin. The amount of cyclin B (which binds to Cdk1) and the activity of the cyclin B-Cdk complex rise through the cell cycle until mitosis, when they fall abruptly due to degradation. The complex of Cdk and cyclin B is called maturation promoting factor (MPF). Other cyclins include cyclin E (binds to G₁ phase Cdk), which is required for the transition from G₁ to S phase and cyclin A (binds to S phase Cdk2) and is required for the cell to progress through the S phase. Maturation promoting factor (abbreviated MPF, also called mitosis-promoting factor) is a heterodimeric protein composed of cyclin (cyclin B, a mitotic cyclin) and cyclin-dependent kinase (CDK1, also known as Cdc2) that stimulates the mitotic and meotic cell cycles. ...

Leland H. Hartwell, R. Timothy Hunt, and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of cyclin and cyclin-dependent kinase, central molecules in the regulation of the cell cycle. Leland H. Hartwell (born October 30, 1939, in Los Angeles, California) is president and director of the Fred Hutchinson Cancer Research Center in Seattle, Washington. ... Dr. R. Timothy (Tim) Hunt (b. ... Sir Paul M. Nurse (b. ... List of Nobel Prize laureates in Physiology or Medicine from 1901 to the present day. ...

References

1. Bai C, Richman R, Elledge SJ. (1994) Human cyclin F. EMBO Journal. 13(24):6087-98.

1. Kong M, Barnes EA, Ollendorff V, Donoghue DJ. (2000) Cyclin F regulates the nuclear localization of cyclin B1 through a cyclin-cyclin interaction. EMBO Journal. 19(6):1378-88.

1. Fung, T. K. and Poon R.Y.C. (2005). A roller coaster ride with the mitotic cyclins. Seminars in Cell & Developmental Biology. 16 (3):335-342. Available via the World Wide Web.
2. Karp G. (2005). Cell and Molecular Biology: Concepts and Experiments, Fourth ed, pp. 148, 165-170, and 624-664. John Wiley and Sons, Hoboken, NJ.