Mitosis

Mitosis

• Mitosis is a nuclear division that gives rise to two genetically identical diploid daughter cells
  • In a human, this diploid number is 23 pairs of chromosomes
• All body cells (and not gametes) are produced by mitosis of the zygote
• Mitosis is required for:
  • Growth: mitosis produces new cells
  • Repair: to replace damaged or dead cells
  • Asexual reproduction: mitosis produces offspring that are genetically identical to the parent

The Cell Cycle

• There are 3 parts of the cell cycle
  • Interphase – Just before mitosis, the DNA in the nucleus copies itself exactly (forms x-shaped chromosomes)
  • Mitosis – Chromosomes line up along the centre of the cell where cell fibres pull them apart
  • Cytokinesis – The cell cytoplasm and membrane divides to produce two daughter cells; each new cell has a copy of each of the chromosomes

The three stages of the cell cycle

The Stages of Mitosis

• The process of mitosis is itself made up of a series of stages that begin after interphase has occurred. These stages occur in the following order:
  • Prophase – DNA condenses, chromosomes become visible and the nuclear membrane breaks down
  • Metaphase – Chromosomes line up on the equator of the cell (the metaphase plate)
  • Anaphase – Fibres split the chromosomes down the centre and pull one chromatid to either side of the cell
  • Telophase – New membranes form around the chromosomes at either end of the cell

The stages of mitosis

Exam Tip

Sometimes you will be asked to calculate the number of cells that would result from a certain number of divisions. In order to work this out, you need to calculate 2 to the power of the number of divisions (2ⁿ).

For example, if we started with 1 cell and it divided 7 times, we would end up with 2⁷ cells, which is 128 cells.