Interphase, a stage of the cellular cycle during which growth and protein synthesis occurs, is the stage in which most cells spend most of their life cycle. This phase can be further broken down into three stages: GAP1 (G1), synthesis (S) and GAP2 (G2). The first stage of interphase, G1, occurs directly after mitosis (M-stage). The cell enters M-stage to undergo chromosomal separation and cytoplasmic division. This phase includes four separate stages called prophase, metaphase, anaphase and telophase and occurs after the last interphase stage, G2.
Most cells spend the majority of their lives in the G1 interphase stage, which is the longest of the interphase stages. Normal functions, such as cellular growth and protein synthesis, are carried out, but deoxyribonucleic acid (DNA) replication has not occurred; only one copy of each DNA molecule exists at this stage. The chromosomes in this stage are not condensed and, therefore, are not visible. Many mature cells, including brain cells, will remain in this stage throughout their lives.
The S-phase of interphase follows the G1-phase. The cell replicates its DNA during this period and, at the completion of this stage, all chromosomes will have two copies of DNA. The chromosomes remain uncondensed during this phase and are not visible.
The cell enters the G2-phase after completing the S-phase. During this second growth phase, the cell continues to carry out normal functions and may continue to increase in size, though most growth occurs in the G1-phase. Cytoplasmic organelles replicate near the end of G2 in order to prepare for cellular division. The first stage of mitosis, prophase, follows the completion of the third interphase stage.
Cyclin dependent kinases (CDKs) and other cyclins help to move the cellular cycle from G1 to S and from G2 to the M-stage by adding a phosphate group to regulatory proteins. Maturation promoting factors (MPFs) are molecules that trigger progression through the cell cycle and include CDKs and cyclins. The proteins p53 and p27 will stop the cellular cycle if DNA is damaged, to inhibit abnormal cell growth, and mutations in these proteins are the most frequent genetic defect that leads to cancer.