Cell Cycle Progression: A Coordinated Act of Cyclins, Cdks and Checkpoints - Research Paper Example

(Pelengaris and Khan, 2013). Go phase is the resting phase which is not usually connected to the cell proliferation. Go phase is celled the quiescent state where it is often concluded as the extension of the G1 phase or as post mitotic phase. G1 phase is the phase of metabolic activity. G1 is the first gap phase where the cell grows and synthesizes various enzymes and metabolites required for the replication of the cell. The duration of this phase varies from species to species. The mature cells stop at the G1 phase and become inactive.

The mature cells will move to the Go phase. The cell cycle regulation and functioning lies in the G1 phase. The cell cycle events are regulated by the independent biochemical control systems called as Check points. (Pelengaris and Khan, 2013). These check points act as a clock and turns on the events at a particular time. Table 1: Phases and Cyclins in the cell cycle: Cell cycle phase Name of the cyclin involved G1 phase Cyclins D1,D2 and D3 , C S phase Cyclins E1, E2 and A ( A1 and A2) Mitotic phase Cyclins B ( B1, B2 and B3) and A.

(Pelengaris and Khan, 2013). Check points enable the cell to trigger each event at the correct order and only once per cycle and contains a variety of signals to check and balance the cell cycle. The cell cycle check points require the cyclin-dependent kinase (Cdks) protein family for the activation and co ordination. There are 11 different Cdks found so far in the eukaryotes. These kinases are the protein molecules that are capable of phosphate groups to other protein molecules through phosphorylation.

(Karp, 2009). Thus kinases activate or deactivate a protein. By doing so, the cell cycle moves to the next phase or stops at the G1 phase itself. Since the cell cycle control is very important, Cdk activity is tightly regulated. Each Cdk protein is responsible for a particular phase of the cell cycle. The phase specific Cdks are controlled by positive and negative regulatory proteins. The positive regulation of the Cdk is dependent on a small regulatory sub protein called as cyclins. The cyclins are synthesized in each phase of the cell cycle.

The gene responsible for the cell cycle in yeast is identified to be cdc2/cdc28. (Karp, 2009). The activation of this gene requires a sub unit called cyclin. The phase specific cyclin associates with the phase specific Cdks and forms a cyclin- Cdk complex. The cyclin box is used for the binding and activation of the Cdks. The increase in the concentration of the specific cyclin triggers the cell cycle. This complex is phosphorylated by the Cdk- activating kinase (CAK). (Karp, 2009). The activation and cycling of the Cdk- cyclin complexes directs the cell cycle events.

The Cdk- cyclin and CAK controls the two important points in the cell cycle. They are the G0/G1 transition and the restriction point. Cdk are the serine/threonine complexes that are found to phosphorylate substrates such as pRb family, p53, B-myb, Cdc25A phosphatase, p27Kip1CkI etc. (Karp, 2009). The activation and inactivation of the Cdk is dependent on the phosphorylation and dephosphorylation of the serine and threonine residues of the cyclin molecule. A protein called mitogen sends a signal to the cell in the G0 phase and the cyclin of D- type is secreted and assembled by the two Cdks.

The Cyclin D1 gene transcription is induced by the Mitogen-induced Ras Signalling molecules. (Pardee and Stein, 2011). Cyclin D act as growth factors sensors by