The Cell Cycle
The cell cycle consists of two main parts. Mitotic phase and interphase. The cell spends most of the cycle in interphase. During interphase, the cell is performing the functions specific to that cell, so interphase happens alongside normal cellular functions.
Stages of Interphase:
G1 phase - cell organelles replicate. This requires a lot of proteins, so there is a high level of transcription/translation of DNA to form the proteins required for this phase. The cell size also increases. This ensures that the daughter cells will be the correct size when formed and not too small.
S phase - all chromosomes are replicated, so the cell synthesises a lot of new DNA. This uses a lot of energy.
G2 phase - The cell replaces the energy stores used in the S phase to replicate the chromosomes. If there is any damage to the chromosomes from replication, it is fixed here. There is more transcription and translation to produce proteins and the size of the cell continues to increase.
Mitotic phase
Comprised of mitosis (when the chromosomes separate into two nuclei) and cytokinesis (when the cell divides into two)
The G0 phase
In the G0 phase, the cells have left the cell cycle. This could be for three reasons:
The cells are fully differentiated and cannot replicate, for example adult neurones.
The cells have irreparable damaged DNA from the S phase, so the cells enter senescence.
The cells are temporarily in G0, for example, B memory cells. These cells can be induced to re-enter the cell cycle if needed.
Checkpoints
There are three checkpoints during the cell cycle. The cell uses these to check that the DNA is not mutated beyond repair. If a cell fails a checkpoint, it will leave the cell cycle and enter the G0 stage.
G1 checkpoint: Cell is checked to be the correct size and for DNA damage.
G2 checkpoint: DNA is checked for damage after synthesis and that the cell is the correct size.
Metaphase checkpoint: Checks chromosomes are assembled correctly on the mitotic spindle, so mitosis can be completed.