Mitosis (AQA AS Biology): Revision Note

The importance of mitosis

• Mitosis is the process of nuclear division producing two genetically identical daughter nuclei

• Each nucleus is genetically identical to the parent nucleus

• Mitosis is fundamental to many biological processes:

Growth of multicellular organisms

• The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell

• This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms

• Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

Replacement & repair of cells

• Damaged tissues are repaired via mitosis, followed by cytokinesis

• Continuous cell loss (e.g. skin, gut lining) requires constant cell replacement

• Some animals show regeneration of body parts (e.g. zebrafish fins, axolotl limbs)

Asexual reproduction

• Involves one parent, producing genetically identical offspring

• In unicellular organisms (e.g. Amoeba), mitosis results in reproduction

• In multicellular organisms, offspring may detach from the parent after growth

  • e.g. runners in strawberries and budding in Hydra and yeast

The stages of mitosis

• Mitosis is divided into four stages:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

• When studying the images below, note that:

  • the diagrams below show mitosis of an animal cell with only four chromosomes. This is for simplicity; in reality, organisms often have many more.

  • different chromosome colours are used to represent the maternal and paternal origin

Prophase

• Chromosomes condense (visible when stained)

• Each chromosome = 2 sister chromatids joined at a centromere

• Centrosomes move to opposite poles

• Spindle fibres (microtubules) form from centrosomes

• Nuclear envelope breaks down

Metaphase

• Centrosomes are located at opposite poles of the cell

• Spindle fibres are fully formed and are attached to the centromeres of the chromosomes

• Chromosomes align on metaphase plate (equator of the cell)

Anaphase

• Spindle fibres shorten, pulling chromatids apart

• Centromeres divide, separating sister chromatids

• Chromatids (now chromosomes) move to opposite poles

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

Recognising the stages of mitosis from images

• Cells undergoing different stages of the cell cycle can be identified using photomicrographs taken from microscope slides

• Cells undergoing certain stages of the cell cycle have distinctive appearances

Recognising prophase

• Chromosomes are visible

• The nuclear envelope is breaking down

Recognising metaphase and anaphase

• Metaphase: Chromosomes are lined up along the middle of the cell

• Anaphase: Chromosomes are moving away from the middle of the cell, towards opposite poles