Cell cycle

The Eukaryotic Cell Cycle

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) Phase

-Here chromosomes are segregated and cell divide. There are 6 stages; prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis.

• G1 (Growth) Phase

-This is the period inbetween mitosis and S-phase. It gives cells time to assess their status and gives them time to enter a quiescent phase called G0, where no cell division occurs. 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. 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.

Cdk and cyclin

Cyclin association with Cytokine dependant kinases (Cdk) activates protein kinase activity.

Cdk phosphorylation of intracellular proteins regulates major events in the cell cycle .

In mammals there are multiple Cdks which function at different stages of the cell cycle:

• G1 PHASE has Cdk4 and Cdk6
• S PHASE has Cdk2
• G2/M PHASE has Cdk1

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