The Cell Cycle & Interphase (AQA AS Biology): Revision Note

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 transition from one phase to another is triggered by chemical signals called cyclins

The length of the cell cycle is variable depending on environmental conditions, the cell type and the organism

• E.g. onion root tip cells divide approximately once every 20 hours and human intestine epithelial cells divide approximately once every 10 hours

During interphase the cell increases in size and carries out normal cellular functions, e.g. synthesising proteins and replicating DNA ready for mitosis

Interphase consists of three phases:

• G₁:

  • G stands for gap

  • Cells make enzymes and other proteins required for growth

  • At some point during G₁ a signal is received, telling the cell to divide again; at this point the cell will progress into S phase

• S phase:

  • S stands for synthesis (of DNA)

  • The DNA in the nucleus replicates, after which each chromosome consists of two identical sister chromatids

  • S phase is relatively short

• G₂ phase

  • The cell continues to grow and the new DNA is checked so that any errors can be repaired

  • Other preparations for cell division are made, e.g. the production of tubulin protein, which is used to make microtubules for the mitotic spindle