HL 1 Biology

2.5b Mitosis

Mitosis: is a process of nuclear division in eukaryotic cells, resulting with 2 genetically identical nuclei. This process is used in eukaryotes when these genetically identical cells are needed during: growth, embryonic development, repair in tissues, to reproduce asexually. Mitosis undergoes the following stages:


  • Early prophase:
    • chromosomes become shorter + fatter by coiling
    • supercoiling: coil repeatedly to become short
  • Late prophase:
    • spindle microtubules extend from opposite sides of equator
    • the chromosomes’ 2 identical chromatids  (from DNA replication) held by centromere
  • *microtubules: grown from poles of cells (from MTOC, microtubule organizing centre) to chromosomes


  • spindle microtubules attach to the centromeres
  • chromosomes move to equator, each sister chromatid is attached to opposite poles


  • pairs of sister chromatids are separate
  • spindle microtubules pull them towards the poles
  • centromeres divided = chromatids become chromosomes

Because sister chromatids pulled to opposite poles = mitosis produces 2 genetically identical nuclei. (chromatids must be combined with spindle microtubules from different poles in metaphase)


  • Early telophase:
    • all chromosomes reach pole
    • nuclear membranes reforms around chromatids (chromosomes now)
    • spindle microtubules break down
  • Late telophase:
    • chromosomes uncoil
    • cell divides (cytokinosis): produce 2 daughter cells
  • start interphase once again


Agents that cause damage to cell cycles:

  • ultraviolet light
  • carcinogens chemicals
  • viruses


  • In any organ/tissue, tumor is formed by occurrence of uncontrolled cell division
  • If tumors infectious: cells can detach/move to other parts of body and develop into secondary tumors


Centromeres and Telomeres (p. 40)

 The cell was in the metaphase stage of mitosis because of the separating chromosomes that are starting to gather at the equator of the cell. It is also evident that the chromosomes has already been coiled because of it’s short and fatter structure.

2 (a14 chromosomes

(b) Body cells in plants and animals have an even number of chromosomes due to the eukaryotes process of reproduction process. The daughter cell, produced from sex, receives 2 DNA sets from the parent cells thus resulting these cells to have 2 (an even number of) chromosomes.

(c) In interphase, the centromeres are on one side of the nucleus and the telomeres are on the other side because they may consist of opposite polarities, resulting these two to repel against each other. This may be a reason for why the centromeres stay in the middle of the chromosome as for the telomeres stick to the ends of the chromatids, far away from one another. This separation between the two may also help make the transition of chromosomes from interphase to prophase smoother.

(d) Consequences for a plant or animal of the shortening of telomeres may include a disrupt issue to the mitosis process and problems with the chromosomes as they may uncoil due to the shortening of the ends.

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