Cell cycle

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This can be described as a distinct and ordered series of events that lead to the replication of a cell, these highly specific events must be highly regulated.  There are two main factors associated with efficient replication of the cell and these must be correct before the cell cycle can commence. 

Firstly the cell must be the correct size if it were to small then after subsequent cell divisions the cell would become too small and become disfunctional. 

Secondly the genetic material must be in the correct state and must only be replcated once per cell cycle. 

Also the cell cycle must only occur in tissues which require the process to take place, uncontrollable cell divsion can lead to the formation of tumours and ultimately formation of cancers. 

The cycle consists of four distinct stages:


M (Mitosis)

Here chromosomes are segregated and cell divide. There are 6 stages; prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis. Only a small period of time of the cell cycle consists of the cell dividing. Mitosis produces two daughter cells of which the chromosome number is the same as the parent and the genome is also kept exactle the same. (Mitosis of a diploid cell will produce two diploid cells and mitosis of a haploid cell will produce two haploid cells.)

G1 (Growth) Phase

This is the period inbetween mitosis and S-phase. It gives cells time to assess their status, acts as a checkpoint and gives them time to enter a quiescent phase called G0, where no cell division occurs. A cell can remain in G0 (a specialized resting state) for days, weeks or even years. Whilst in G1, the cell increaes in size, reproduce RNA and synthesise proteins. There is an important cell cycle control mechanism activated in this phase that monitors the cell size and determines whether the cell has damaged DNA. If the cell is not an appropriate size and has damaged DNA, progress through the cell cycle stops until it has been corrected.

S (Synthesis) Phase

This is where chromosome replication takes place, this can take up to 8 hours(in a typical human cell). It is crucial that the genetic material is only replicated once per cell cycle.

G2 (Growth) Phase

This is a period between s-phase and mitosis. It allows the cell to ensure all replication is complete before commiting to mitosis. In G2 Phase the cell continues to grow and produce new proteins which are required for when the cell divides. At the end of this phase is the second checkpoint; this is when the cell checks the physiological conditions before it proceeds into mitosis. Again, the cycle will not continue if the conditions are not correct or repair needs to take place. By the end of G2 the volume fo the cell has doubled, DNA has been replicated and therefor mitosis is initiated.

Cdk and cyclin

Cdk's are composed of a regulatory cyclin subunit and a catalytic cyclin-dependent kinase (CKD)subunitt [1]. Cyclin association with Cytokine dependant kinases (Cdk) activates protein kinase activity.

Cdk phosphorylation of intracellular proteins regulates major events in the cell cycle [2]. In the later stage of the G1 phase, cyclin-CDK complexes induce degradation of S-phase inhibitors by phosphorylation. This activates S-phase cyclin-CDK complexes.The initiation of DNA replication is brought about by the activated S-phase cyclin-CDK complexes phosphorylating regulatory sites in proteins related to DNA replication. This phosphorylation also prevents reassembly of new pre-replication complexes meaning chromosomes are replicated just once. In mammals there are multiple Cdks which function at different stages of the cell cycle  [3].

These Cdk's form Cyclin-Cdk complexes with cyclins. In mammals;

- Cdk4 and Cdk6 can join with Cyclin D1, Cyclin D2 or Cyclin D3

- Cdk2 can join with Cyclin E or Cyclin A

- Cdk1 can join with Cyclin B


  1. Lodish, H., 2004. Molecular Cell Biology. 5th ed. s.l.:WHFreeman
  2. Alberts, B et al. (2002) Molecular Biology of the Cell (4th ed) New York: Garland Science
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