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HBIO2 > Cell Division
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Cell Division

  • Mitosis
    • Part of a larger cell cycle
    • Occurs in the nucleus of eukaryotic cells
    • Responsible for growth and repair
      • Replaces the entire lining of the small intestine
      • Liver cells only divide for repairing
      • Nerve cells do not divide
    • New cells arise by division of existing cells
    • Resulting cell is capable to divide again
  • Meiosis
    • Cell divides twice to becomes a gamete
    • Will not divide any further until fertilization


  • Humans have 46 chromosomes
  • Made up of 2 identical chromatids which are connected by a centromere
  • Contain DNA
  • Long and thin for replication and decoding
  • Become short and fat prior mitosis → easier to separate due to compact form

Cell Cycle

  • Interphase
    • Phase with highest metabolism (mitochondria have a high activity)
    • Muscles never complete the whole cycle
    • G1: Protein synthesis and growth (10 hours)
      • Preparation for DNA replication (e.g. growths of mitochondria)
      • Differentiation, only selected genes are used to perform different functions in each cell
    • S: DNA Replication (9 hours)
    • G2: short gap before mitosis to allow organelles and proteins for mitosis to be made (4 hours)
  • G0: Resting phase (nerve cells)
  • M-phase (mitosis)
    • Mitotic division of the nucleus (prophase, metaphase, anaphase, telophase)
    • Cytokinesis (division of the cytoplasm)


  • Process of producing 2 diploid daughter cells with the same DNA by copying their chromosomes (clones)
  • Chromosomes can be grouped into homologous pairs
  • Mitosis occurs in
    • Growth
    • Repair
    • Replacement of cells with limiting life span (red blood, skin cells)
    • Asexual replacement
  • Controlled process, cancers result from uncontrolled mitosis of abnormal cells
  • Division of the nucleus (karyokinesis) and the cytoplasm (cytokinesis) are two processes of mitosis
  • Division of cytoplasm after nucleus. Delayed if cells have more than one nucleus (muscle)
  • Active process that requires ATP


  • Chromosomes become shorter and thicker by coiling themselves (condensation)
  • This prevents tangling with other chromosomes
  • Nuclear envelope disappears/breaks down
  • Protein fibres (spindle microtubules) form
  • Centrioles are moving toward opposite poles forming the spindle apparatus of microtubule


  • Centrioles at opposite poles
  • Chromosomes line up on the equator of the spindle
  • Centromeres (kinetochores) attach to spindle fibres
  • Kinetochores consist of microtubules and "motor" proteins which utilise ATP to pull on the spindle


  • Spindle fibres separate and pull copies of chromatids to spindle poles
  • Mitochondria around spindle provide energy for movement


  • Chromatid at the pole
  • Sets of chromosomes form new nuclei
  • Chromosomes become long and thin, uncoil!
  • Nuclear envelope forms around the nucleus


  • Region of constriction
  • Actin molecules (ring) contract


  • Sex and cell division - the production of gametes
  • DNA in a cell replicates only once, but cell divides twice
    • First division
      • Random segregation of homologous chromosomes
      • This produces 2 haploid cells
      • Chromosomes are still duplicated (made up of 2 chromatids)
    • Second division
      • Random segregation of chromatids
      • Centromeres divide
      • Chromatids are pulled to opposite sides
      • This produces 4 haploid gametes → sperm/ovum
  • Diploid cell (2n = 46 chromosomes) produces 4 haploid daughter cells (n = 23 chromosomes)
  • Fertilisation of sperm (n = 23) and ovum (n = 23) produces one zygote (2n = 46)

Down's Syndrome

  • Caused by non-disjunctionmistake in meiosis produces an extra chromosome 21
  • Pair of chromosomes of one parent fails to separate during second division of meiosis
    • Chromatids fail to segregate in one haploid cells
      • One gamete has NO chromosome 21
      • Other gamete has 2 chromosomes 21
    • Other haploid cells produce 2 normal daughter cells/gametes
      • They each contain one chromatid
  • Fertilization
    • Normal gamete (from partner) fuses with abnormal gamete that contains 2 chromosome 21s
    • Zygote ends up with 3 copies of chromosome 21!
    • Total chromosome count is 47 rather than 46!
  • [Background Knowledge]
    • Risk increases with age
    • Triple test during pregnancy screens babies for Down’s syndrome
    • Only shows the risk of developing Down’s syndrome - not a diagnosis!
    • Diagnosis by amniocentesis - allows early abortion
  • Clinical features
    • Learning difficulties
    • Short stature
    • Different physical appearance
    • Severity of symptoms varies from person to person