Cell division

• Cell cycle phases


  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Interphase


  • Division
  • Cytokinesis
• Cytokinesis stages


  • A
  • B
  • C
  • D
• Mitosis

  • Produces cells which are genetically identical to the parent cell, giving genetic stability
• Reasons for cell division

  • Growth
  • Repairing damaged cells
  • Replacement of old, worn-out cells
  • Asexual reproduction
• Cell cycle control


  • Genes regulate the cell cycle
  • If genes are damaged, uncontrolled mitosis can occur
  • Rapid replication of cells can form tumours, leading to cancer
  • Genes that cause cancer are called oncogenes
• Cytokinesis


  • Division of the cytoplasm to create 2 new genetically identical cells
• Plant cell cytokinesis


  • No centrioles
  • Cell plate (droplets of cell wall material) develops from the centre out instead of cleavage from the outside in as in animal cells
• Prophase


  • Chromosomes condense and become visible
  • Centrioles move to opposite sides of cell in animal cells
  • Spindle forms from microtubules
  • Nuclear envelope disintegrates
• Metaphase


  • Centromeres of chromosomes attach to spindle and line up on the equator
• Anaphase


  • Spindle fibres shorten
  • Centromere separates and individual chromatids are pulled to the poles centromere first
• Telophase


  • Spindle breaks down
  • Chromosomes uncoil
  • Nuclear envelope reforms
• Interphase


  Time of high metabolic activity for the cell
• Interphase section A


  • Replication of organelles such as mitochondria and chloroplasts which have their own DNA
  • New organelles are made
  • Synthesis of ATP and proteins
  • Increase in cell size
• Interphase section B


  DNA replication
• Meiosis


  • Takes place in gonads
  • Produces cells which are haploid for sexual reproduction (gametes)
• Meiosis first division


  • Differs from prophase in mitosis as chromosomes form bivalents (pairs of homologous chromosomes)
  • Arms of the chromatids may cross over forming chiasmata, increasing variation in inherited genomes
• Meiosis second division


  • Homologous chromosomes arrange themselves in pairs along the equator
  • Independent assortment occurs where homologous chromosomes from parent 1 and parent 2 arrange themselves randomly along the spindle facing each pole
  • The chromosome bivalents separate as each chromosome is pulled by its centromere towards the opposite pole
  • Nuclear envelopes reform around the chromosomes at the poles
  • Prophase II occurs after telophase I
  • Chromosomes are no longer in pairs and the cells are haploid
• Comparing mitosis and meiosis


  • Number of nuclear divisions in the process
  • Number of cells formed
  • Ploidy of parental cells/nuclei
  • Ploidy of daughter cells/nuclei
  • Genetic nature of daughter cells/nuclei
  • Pairing of homologous chromosomes
  • Crossing over
  • Segregation of homologous chromosomes
• Haploid cells can only divide by mitosis
• Cell cycle phases


  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Interphase


  • Division
  • Cytokinesis
• Cytokinesis stages


  • A
  • B
  • C
  • D
• Mitosis


  • Produces cells which are genetically identical to the parent cell, giving genetic stability
• Reasons for cell division

  • Growth
  • Repairing damaged cells
  • Replacement of old, worn-out cells
  • Asexual reproduction
• Cell cycle control


  • Genes regulate the cell cycle
  • If genes are damaged, uncontrolled mitosis can occur, leading to tumours and cancer (oncogenes)
• Mnemonic IPMAT


  Remembers the order of mitosis phases (Interphase, Prophase, Metaphase, Anaphase, Telophase)
• Cytokinesis


  Division of the cytoplasm to create 2 new genetically identical cells
• Differences in plant cell cytokinesis

  • No centrioles
  • Cell plate develops from centre out instead of cleavage from outside in
• Prophase


  1. Chromosomes condense and become visible

  2. Centrioles move to opposite sides of cell in animal cells

  3. Spindle forms from microtubules

  4. Nuclear envelope disintegrates
• Metaphase


  Centromeres of chromosomes attach to spindle and line up on the equator
• Anaphase


  1. Spindle fibres shorten

  2. Centromere separates and individual chromatids are pulled to the poles centromere first
• Telophase


  1. Spindle breaks down

  2. Chromosomes uncoil

  3. Nuclear envelope reforms
• Interphase


  • Time of high metabolic activity: Replication of organelles, new organelles made, ATP and protein synthesis, increase in cell size
  • DNA replication
• Meiosis


  • Produces haploid cells for sexual reproduction (gametes)
  • Differs from mitosis in first division: Chromosomes form bivalents, chiasmata allow genetic exchange, independent assortment of homologous chromosomes
• Meiosis I


  1. Prophase I: Chromosomes form bivalents, chiasmata form

  2. Metaphase I: Homologous chromosomes arrange in pairs

  3. Anaphase I: Bivalents separate, chromosomes move to poles

  4. Telophase I: Nuclear envelopes reform
• Meiosis II


  1. Prophase II: Chromosomes no longer in pairs

  2. Metaphase II: Chromosomes line up

  3. Anaphase II: Chromatids separate, move to poles

  4. Telophase II: Nuclear envelopes reform
• Cytokinesis occurs after meiosis II, resulting in 4 genetically varied cells
• Haploid cells can only divide by mitosis