The cell cycle includes three main stages: interphase, mitosis and cytokinesis. During the activities of interphase, the cell monitors its environment and prepares itself for the next division. Mitosis and cytokinesis represent the stages when duplicated contents physically separate into two separate cells. Interphase activity is further broken down into three stages: G1, S and G2. Cells spend most of their lifespan in the interphase stages.
The cell cycle begins with G1, or first gap, which is the time between birth of a new cell and the beginning of DNA replication. During G1, the cell grows in size and begins protein synthesis and organelle – specialized and organized structures – duplication. The cell monitors its environment to ensure that external conditions are suitable for cell division. The first internal checkpoint on cell division also occurs in the G1 phase, verifying that DNA is not damaged before proceeding to the DNA duplication step.
After passing through the first internal checkpoint, the cell enters the S phase, or synthesis phase. During the S phase, DNA is duplicated so that each chromosome contains two identical copies. These DNA copies do not separate into duplicate chromosomes until after the interphase stages end and mitosis or cell nucleus division begins. The S phase ends when the total amount of DNA has doubled and the cell enters the G2 phase.
During the second gap, or G2 phase, the cell continues to grow in size and produce proteins necessary for cell division. Microtubules, filamentous internal structures, necessary for separating the chromosome copies are made during this phase. The second internal checkpoint that determines if the cell can continue through its cycle occurs in G2. While the G1 checkpoint checks to verify DNA is error free before replication, the checkpoint in G2 confirms the new DNA is error free after replication.
Cells can also pause interphase during G1 and enter a resting state called G0. If the conditions outside the cell do not support division, the cell pauses in G0 for an indeterminate amount of time before resuming the cell cycle and normal growth. If conditions change, the cell can either move from G0 back into the G1 phase, or become post-mitotic and leave the cell cycle completely.