The Gap 1 phase, or G1 phase, is like a growth period for the cell before DNA replication happens. During the G1 phase, the cell functions normally as it grows to nearly double its size. The cell also creates new proteins and monitors intracellular activity to ensure that the cell is healthy. Once the cell determines it is ready for DNA replication, it enters into the next phase, synthesis.
During synthesis, cells replicate their DNA, which is packaged in chromosomes. DNA replication is significant because it allows for identical cells to be formed during cell division. Cell growth continues and the number of enzymes needed to perform DNA replication increases. Synthesis is complete when the number of chromosomes has doubled, meaning the cell contains two complete, identical sets of DNA.
The Gap 2 phase, also known as the G2 phase, is like a checkpoint in the cell life cycle. During the G2 phase, the cell ensures that it is ready for mitosis. The cell checks that all DNA is properly replicated and continues to grow. The cell creates various proteins, like kinase, that are needed for cell division. Once the G2 phase is complete, the cell is ready to begin cell division.
Occasionally cells will not be ready to enter the synthesis phase at the end of the Gap 1 phase. Cells might not be ready to duplicate their DNA because of a lack of nutrients. When this happens, the cell can enter into a phase called the Gap 0, or G0, phase. Some cells will mature and remain in the G0 phase forever, while others will remain in the G0 phase until they are ready to enter synthesis.