Interphase (AQA AS Biology) : Revision Note

The cell cycle

• 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

Interphase

• 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

Nuclear division (e.g. mitosis)

• Nuclear division follows interphase

• In normal body cells this will be mitosis, while in the production of gametes this will be meiosis

Cytokinesis

• Division of the cytoplasm follows nuclear division

• Once the nucleus has divided into two genetically identical nuclei, the whole cell divides and one nucleus moves into each cell to create two genetically identical daughter cells

  • In animal cells cytokinesis involves constriction of the cytoplasm between the two nuclei

  • In plant cells a new cell wall is formed