MoBio Cell Cycle Chapter 8

The cell cycle is a series of events that lead to cell division. It consists of four phases: G1, S, G2 and M, where "G" stands for "gap", "S" represents "synthesis" and "M" means "mitosis". A newly divided cell may either enter into another round of cell division or remain in the resting state for a long period of time. In the latter case, the cell is said to be in the G0 phase. Upon specific stimulation, cells in the G0 phase may re-enter the G1 phase.

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Figure 8-A-1. The cell cycle. Without counting the G0 phase, a cell cycle takes 12-24 hours for most mammalian cells, and only 20-30 minutes for E. coli cells.

CDK and Cyclins

The progression of a cell cycle is catalyzed by cyclin-dependent kinase (CDK) which, as the name suggests, is activated by a special class of proteins called cyclins. In mammals, different cyclins are designated as A, B, C, D and so on. In budding yeast (S. cerevisiae), nine cyclins have been identified: Cln1 to Cln3 and Clb1 to Clb6.

The proteins of the CDK superfamily in mammals is denoted by Cdk followed by a number. However, in yeast, they are denoted by Cdc (cell division control) and a number. Among them, Cdc2 was first identified in fission yeast ( S. pombe). It is equivalent to Cdc28 in budding yeast, and to Cdk1 in mammals. Cdk1 is often referred to as Cdc2.

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Figure 8-A-2. Structure of the Cyclin A-Cdk2 complex. PDB ID = 1FIN.

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Figure 8-A-3. The cyclin-CDK complexes involved in each phase of the cell cycle. The red line indicates the time a particular complex is involved. In budding yeast, the cell cycle is initiated by the binding of Cln1 or Cln2 to Cdc28. Then, Clb5 or Clb6 binds to Cdc28, and pushes the cycle forward. Clb1 and Clb2 are involved in the final phase. In mammals, the cell cycle is initiated by the binding of Cyclin D (CycD) to Cdk4 or Cdk6. The final phase is catalyzed by the binding between Cyclin B (CycB) and Cdk1. The CycB-Cdk1 complex is known as MPF, which originally stands for maturation promoting factor, but now also stands for mitosis promoting factor.

Related Sections

Initiation and termination mechanism of the S phase

p53 and cell cycle control