View Cell Cycle Figure.

Quiescence: Cells which are not proliferating are said to be quiescent or in "G0" phase.  The metabolic demands of G0 phase varies according to the specialized functions being carried out by the quiescent cell, but there is often lower levels of gene expression, macromolecular biosynthesis and energy consumption compared to actively dividing cells. 

G1 Phase: During the first phase (G1) cells grow in size in response to mitogenic signals, such as soluble extracellular growth factors and intracellular contact, which may trigger a commitment to entering the next phase of the cell cycle. During G1 protein kinases, enzymes which catalyze the addition of phosphate other proteins, become active and thereby send a signal that the cell division process has begun. The cyclin dependent kinases (CDKs) are a group of such protein kinases whose activity is central to this process. The CDKs are so named because their activity requires binding to a protein partner, a cyclin, whose levels of expression and activity varies depending on the phase of the cell cycle. The first group of cyclins to become activated are the D-type cyclins which bind and activate CDK4 or CDK6. The active cyclin D/CDK4-6 enzyme phosphorylates Rb, the retinoblastoma protein, which impedes its ability to bind and inactivate transcription factors. These factors, such as E2F-1, when released from Rb, bind to the promoter regions of specific genes resulting in increased RNA transcription.

The Restriction Point: Late in G1, many cell types become committed to entering the next phase of the cell cycle at a time termed the restriction point. The existence of a restriction point can be demonstrated by the simple act by depriving cells from growth factors in their medium.  Many types of cells will continue to complete a single cell cycle, if they are in S-phase, G2 or Mitosis, but will then arrest in the next G1 phase.  Activation of a second class of cyclins, the E-type cyclins, together with a CDK catalytic partner, may act as the restriction point switch.

S-Phase: Soon after the restriction point a cell begins to replicate its genetic material through the activity of a family of enzymes including the DNA polymerases, which are capable of synthesizing an exact replica of the DNA genome. In a mere 6-hours the cell polymerizes exact replicas of all 46 chromosomes, totalling six billion nucleotides, while maintaining the identical sequence of nucleotides as the parental DNA and thus preserving the genetic information. This "S-phase" of the cell cycle is thus biochemically distinct from the other phases of the cell cycle and can easily be distinguished from other cells by their ability to stably incorporate fluorescently or radioactively tagged nucleotides, the building blocks of DNA, into their chromosomes. To activate the replication machinery cells depend on the protein kinase activity cyclin A coupled to CDK1 and CDK2.

G2 Phase: At the completion of S-phase, DNA replication ceases and cells enter the G2 phase of the cell cycle. During this phase CDK1 (a.k.a. cdc2) replaces CDK2 as the predominant cyclin and it couples with either Cyclin A or one of the B-type cyclins to catalyze the phosphorylation of proteins specific to the G2 and M phases of the cell cycle.

Mitosis: Mitosis is the phase of the cell cycle in which cells physically divide into two separate daughter cells. In order to do so they first dissolve the nuclear membrane which will later reform once cell division is complete. The DNA-containing chromosomes then condense into structures so compact that they can seen with a light microscope. The chromosomes then precisely segregate to two sides of the cell, such that each half of the cell gets exactly one copy of each chromosome. At the completion of mitosis, cells undergo cytokinesis or separation into two halves. This occurs as a band forms around the circumference outer plasma membrane which gradual constricts like a belt until the cell pinches in two. The B-type cyclins remain active throughout M-phase, but their activity immediately ceases once cell division is complete and the two daughter cells once again enter G1.