5-1-replication-and-division-of-nuclei-and-cells AS the-cell-cycle

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 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

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