Interphase
- Mitosis is part of a precisely controlled process known as the cell cycle
- The cell cycle is the regulated sequence of events that occurs between one cell division and the next
- The cell cycle has three phases:
- interphase
- nuclear division (mitosis)
- cell division (cytokinesis)
- The length of the cell cycle varies depending on:
- The environmental conditions, the cell type and the organism
- For example, onion root tip cells divide once every 20 hours (roughly) but human intestine epithelial cells divide once every 10 hours (roughly)
- The movement from one phase to another is triggered by chemical signals called cyclins
The cell cycle
S = synthesis (of DNA); G = growth; M = mitosis
Interphase
- Interphase is the longest and most active phase of the cell cycle
- During interphase, the cell:
- Increases in mass and size
- Carries out many cellular functions in the nucleus and cytoplasm eg. synthesising proteins and replicating its DNA ready for mitosis (these only occur during interphase)
- Increases the number of mitochondria
- Increases the number of chloroplasts (if they are a plant or algae cell)
The phases of interphase
- Interphase consists of three phases:
- G1 phase
- S phase
- G2 phase
- The gap between the previous cell division and the S phase is called the G1 phase – G stands for growth
- Cells make the RNA, enzymes and other proteins required for growth during the G1 phase
- It is at some point during the G1 phase a signal is received telling the cell to divide again (although some cells do not receive this signal and will never divide; they enter the G0 phase)
- After the G1 phase of interphase the cell enters the next phase of the cell cycle, the S phase – S stands for synthesis (of DNA)
- The S phase is relatively short
- The DNA in the nucleus replicates, resulting in each chromosome consisting of two identical sister chromatids
- Between the S phase and next cell division event the G2 phase occurs
- During the G2 phase, the cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired
- Other preparations for cell division are made (eg. production of tubulin protein, which is used to make microtubules for the mitotic spindle)
- Interphase = G1 + S + G2
Examiner Tip
Make sure you know the order of the phases of the cell cycle but also what specifically occurs during the different phases. Don’t forget, interphase is itself made up of three distinct stages (G1, S and G2) and you need to know what happens during each of these.For example, an exam question might ask you to identify the stage of the cell cycle during which a cell would be producing the most mRNA molecules and explain why. The correct answer would be the G1 phase, as this is when protein synthesis is occurring and the production of mRNA occurs during transcription (the first part of protein synthesis).