Interphase is the longest phase of the cell cycle and has three subphases: G1, S, and G2. G1 involves growth and protein/organelle production, followed by a checkpoint. S phase involves chromosome duplication, and G2 involves final preparations for division and another checkpoint. After mitosis, cells return to G1 and repeat the cycle until apoptosis.
Interphase is the first phase of the cell cycle. It directly precedes mitosis, or cell division, and is the state in which a cell spends most of its life. Specific subphases of interphase include the first gap phase (G1), synthesis (S), and the second gap phase (G2).
At the beginning of G1, the cell undergoes a normal period of growth. During this sub-phase, many proteins and various organelles are produced, and the cell usually increases in size. Cells can remain in interphase for very long periods of time.
Step G1 ends with a checkpoint. Checkpoints are used in the cell cycle to ensure that only healthy cells without mutations are duplicated during the division process. Cells that fail the checkpoint are put into gap zero (G0) phase. Most cells in the human body are in G0 at any given time. They can finish their lives without dividing, or they can be recalled into mitosis when needed.
If the cell passes the checkpoint, it moves to S phase. During S phase, cells duplicate their chromosomes. Chromosomes carry all the genetic information that controls the vital processes of the cell. In preparation for division, chromosomes must make an exact copy of themselves. These copies will occupy the new cell that results after the split.
After the chromosomes are successfully duplicated, the cell moves into the G2 subphase. This final stage of interphase includes increased protein production and the creation of organelles. All split preparations must be completed during G2. The organelles and cytoplasm created will be equally divided between the cells during mitosis.
G2 ends with another checkpoint. Before the cell can continue the cell cycle into the mitotic phase, enzymes – such as protein kinases – “reread” the cell’s deoxyribonucleic acid (DNA) to check for errors. Cells that fail this inspection will go to G0. If the enzymes find no errors, the cells can then switch to mitosis and replicate.
After cytokinesis of the mitotic stage, the newly divided cells will return to G1 of interphase and begin the cycle again. They will have to pass the checkpoint inspections again before moving on from the interphase. This will continue until the cell undergoes apoptosis or programmed cell death.
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