- G1 Phase: Also known as”GAP 1” stage this phase of the cell cycle during which the cell increases its biosynthetic rate. During this phase a cell will grow larger in size and increase its supply of proteins and organelles (e.g. mitochondria, ribosomes).
- S Phase: This is the synthesis phase when chromosomal DNA replication occurs. At the conclusion of this phase the DNA of the cell has doubled.
- G2 Phase: Also known as the “GAP 2” this phase of the cell cycle. During this phase of the cell cycle the cell will continue to grow. Check point control mechanisms ensure that the DNA has been properly replicated and is that none of the DNA damage detection and repair mechanisms need to be activated before the cells divide.
- M Phase: The Mitotic phase of the cell cycle described the process by which a eukaryotic cell separates its chromosomes into two separate nuclei. This requires the chromosomes condense and the centromeres to attach to fibers so that the sister chromatids can be pulled to opposite poles of the cell. Cytokinesis typically follows this event and the nuclei, cytoplasm, organelles and cell membrane divide into two cells that contain similar amounts of these components.
Progress through the cell cycle is controlled by cyclins and cyclin-dependent kinases (CDKs). These various cyclins and CDK proteins function as pairs and act as control switches that will allow a cell to move from G1 to S or G2 to M.
The various cyclin/cdk complexes that regulate e progression through the cell cycle are activated by the synthesis of specific cyclins during a discreet phase of the cell cycle. Cyclins have no catalytic activity and CDKs are inactive in the absence of their partner cyclin. When activated by a cyclin, sequential phosphorylation and dephosphorylation of a variety of target protein complexes occurs to orchestrate coordinated passage through the cell cycle.