The Cell Cycle (CIE A Level Biology)

• 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 is very variable depending on 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 Diagram

The cell cycle consists of different phases. Note the following abbreviations: S = synthesis (of DNA); G = gap; M = mitosis

Interphase

• During Interphase the cell increases in mass and size and carries out its normal cellular functions (e.g. synthesising proteins and replicating its DNA ready for mitosis)
• Interphase consists of three phases:
  • G₁ phase
  • S phase
  • G₂ phase
• It is at some point during the G₁ phase that a signal is received telling the cell to divide again
• The DNA in the nucleus replicates (resulting in each chromosome consisting of two identical sister chromatids)
• This phase of the interphase stage of the cell cycle is called the S phase – S stands for synthesis (of DNA)
  • The S phase is relatively short
• The gap between the previous cell division and the S phase is called the G₁ phase – G stands for gap
  • Cells make the RNA, enzymes, and other proteins required for growth during the G₁ phase
• Between the S phase and the 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 (e.g. production of tubulin protein, which is used to make microtubules for the mitotic spindle)
• Interphase = G₁ + S + G₂

Nuclear division (mitosis)

• Follows interphase
• Referred to as the M phase – M stands for mitosis
• Cell growth stops during the M phase
• During mitosis, the nucleus divides to form two genetically identical nuclei

Cytokinesis

• Follows M phase
• 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 and in plant cells a new cell wall is formed