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First exams 2025

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Mitosis (HL IB Biology)

Revision Note

Marlene

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Marlene

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Phases of Mitosis

  • Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)
  • Significance of mitosis: mitosis occurs whenever the production of genetically identical nuclei are required in eukaryotic cells
    • E.g. during embryonic development, growth, tissue repair and asexual reproduction

Phases of Mitosis

  • Although mitosis is, in reality, one continuous process, it can be divided into four main stages or phases

  • These stages are:
    • Prophase
    • Metaphase
    • Anaphase
    • Telophase
  • Most organisms contain many chromosomes in the nuclei of their cells (eg. humans have 46) but the diagrams below show mitosis of an animal cell with only four chromosomes, for simplicity
  • The different colours of the chromosomes are just to show that half are from the female parent and half from the male parent

Prophase

  • Chromosomes condense and are now visible when stained
  • The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
  • The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
  • Spindle fibres (protein microtubules) begin to emerge from the centrosomes (consists of two centrioles in animal cells)
  • The nuclear envelope (nuclear membrane) breaks down into small vesicles
  • The nucleolus disappears

Prophase of mitosis, downloadable AS & A Level Biology revision notes

Prophase

Metaphase

  • Centrosomes reach opposite poles
  • Spindle fibres (protein microtubules) continue to extend from centrosomes
  • Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
  • Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
    • This attachment involves specific proteins called kinetochores
  • Each sister chromatid is attached to a spindle fibre originating from opposite poles

Metaphase of mitosis, downloadable AS & A Level Biology revision notes

Metaphase

Anaphase

  • The sister chromatids separate at the centromere (the centromere divides in two)
  • Spindle fibres (protein microtubules) begin to shorten
  • The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Anaphase of mitosis, downloadable AS & A Level Biology revision notes

Anaphase

Telophase

  • Chromosomes arrive at opposite poles and begin to decondense
  • Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
  • The spindle fibres break down
  • New nucleoli form within each nucleus

Telophase of mitosis, downloadable AS & A Level Biology revision notes

Telophase

Examiner Tip

Make sure you learn the four stages of mitosis and what is happening to the DNA molecules (one chromatid contains one DNA molecule) at each stage – learn ‘PMAT’ (prophase, metaphase, anaphase, telophase) to help you remember the order of the stages!

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.