Cells must divide to allow an organism to grow and for tissues to be repaired. The content of cells is not fixed. Before cells can divide, they must grow larger, produce new organelles and replicate DNA. The process of growth, synthesis and division occur in a particular order known as the cell cycle.

There are 3 phases in the cell cycle:

1. Interphase: Cells grow and synthesise new proteins, produce organelles and replicate DNA in preparation for nuclear and cell division.

2. Mitosis: Nuclear division to produce two genetically identical nuclei.

3. Cytokinesis: Division of cytoplasm to produce two daughter cells.

In mitosis, the daughter cells are genetically identical to the parent cell and to each other. It’s important to remember that they are not just identical as they may contain different numbers of organelles.

Interphase is further split into 3 phases:

1. G1 (first growth) phase: The cell grows in size and synthesises more proteins and membranes and produces more organelles. The cellular contents, excluding chromosomes, are duplicated.

2. S (synthesis) phase: The DNA is replicated.

3. G2 (second growth) phase: Cell growth and protein synthesis continues, and the replicated DNA is checked for any errors. The cell also prepares for mitosis as centrioles replicate.

Control of the cell cycle

Progression through the cell cycle is tightly controlled as uncontrolled cell division will lead to the formation of a tumour. There are 3 major cell cycle checkpoints. At a checkpoint, progression through the cell cycle is halted until all necessary conditions are met.

The 3 checkpoints are:

1. G1/S checkpoint: Ensures any DNA damage is repaired, the cell is large enough and the environment is suitable for the cell to divide (otherwise the cell may enter G0).

2. G2/M: Checks the DNA has been accurately replicated and repairs any errors. The cell will enter mitosis if it is large enough and the environment is suitable.

3. M checkpoint: Ensures all chromosomes are attached to the mitotic spindle.

The order and timing of the cell cycle is tightly controlled. Proteins called cyclins are produced, which activate enzymes called cyclin-dependent kinases (CDKs). These enzymes are activated at particular checkpoints in the cell cycle and control each step. Different cyclins initiate different phases of the cell cycle. The cell cycle can be stopped by the production of other proteins that bind to a CDK. For example, if the DNA is damaged, a protein called p21 is produced which binds to a CDK to stop the cell cycle at the G1/S checkpoint. This allows the DNA to be repaired, before it is replicated in the S phase. If the DNA is too damaged to be repaired, then apoptosis (programmed cell death) is triggered. A failure to repair damaged DNA could lead to uncontrolled cell division resulting in a tumour.

G0 phase

G0 is a resting phase when a cell leaves the cell cycle. In this phase a cell may undergo apoptosis or senescence (loss of a cell's power of division and growth). It may be temporary e.g. if the environment is unsuitable for division, or permanent e.g. if the cell is fully differentiated.