G1 Phase

G1 is the first gap phase of the cell cycle, occurring immediately after a cell has finished dividing. At this point the cell's DNA content is at its unreplicated 2C level. During G1 the cell becomes metabolically very active: it grows in size by synthesising proteins, lipids and carbohydrates, and it builds new copies of cell organelles, including mitochondria and endoplasmic reticulum, so that the eventual daughter cells will each be a fully equipped, functional cell rather than a stripped-down fragment. G1 is also the phase most tightly monitored by the cell's internal quality-control system: many checkpoints operate here to check that the cell is large enough, has adequate nutrients and growth-factor signalling, and has undamaged DNA before allowing it to commit irreversibly to replicating its genome. The most important of these is the restriction point near the end of G1, which acts as the point of no return - once a cell passe …