The interphase is a much longer phase of the cell cycle than the relatively short M phase. During interphase the cell prepares itself for the process of cell division. Interphase is divided into three subphases: G1 (first gap), S (synthesis), and G2 (second gap). During all three parts of interphase, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G1), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G2), and finally divides (M) before restarting the cycle. All these phases in the cell cycle are highly regulated by cyclins, cyclin-dependent kinases, and other cell cycle proteins. The phases follow one another in strict order and there are cell cycle checkpoints that give the cell cues to proceed or not, from one phase to another. Cells may also temporarily or permanently leave the cell cycle and enter G0 phase to stop dividing. This can occur when cells become overcrowded (density-dependent inhibition) or when they differentiate to carry out specific functions for the organism, as is the case for human heart muscle cells and neurons. Some G0 cells have the ability to re-enter the cell cycle.
The interphaSe iS a mucH LoNgER phase of tHE cell cyCle than thE ReLativElY shOrt m pHasE. during iNtErpHAse THe celL PrEpaREs ItSELf for THE prOCeSs Of Cell divIsion. interphase iS diVideD iNTo three subPhases: g1 (fIrSt gAp), s (SYnThesis), AnD G2 (secOnd gap). During All thREe PARts OF inTeRphasE, The ceLL groWs by ProducinG PrOtEins ANd cytOplasMIc orGanelLES. however, chromosoMes ArE rEPLicaTEd onLy DUriNg The s PhaSe. thuS, a cell gRows (g1), contInUes tO GrOW as IT DuplicateS itS Chromosomes (S), gRows mOrE aNd prepares For mitOsIs (G2), aNd Finally dividEs (m) befOre rEstArtIng The CYClE. all thEsE Phases in the celL cYcLE are higHLY ReguLAtEd by CYcLInS, cyclIn-DePeNdeNt kInases, And otHeR ceLl cyCLE protEins. THe PHASeS follow One ANother in sTrIct oRder And theRe are cELl cycle checkPOInts that Give thE celL cuEs TO procEed oR not, fRom oNe PHasE to AnoThEr. cElls may AlsO teMpoRaRIlY Or perManentlY Leave ThE celL CycLe ANd entEr G0 pHaSe tO stoP dividInG. This Can ocCUr when celLS Become oveRcrowdeD (deNsitY-depeNdenT iNhiBiTion) or When They DiffeRentiATe tO caRrY out sPeciFic FUnCtIons FoR thE oRGanIsm, as is thE cAse for humAn HearT musclE cellS and neuroNs. Some g0 ceLls haVe thE ability to rE-EnteR the CelL cYclE.