Module 2: Mitosis and Cytokinesis
These free OCR A Level Biology Mitosis and Cytokinesis revision notes have been written for specification points 2.1.6(c) and 2.1.6(e).
Mitosis
Mitosis is the process of nuclear division in eukaryotic cells and consists of four stages:
- Prophase
- Metaphase
- Anaphas
- Telophase
Mitosis produces two genetically identical daughter cells; they each have the same (and number of) chromosomes.
The table below outlines the stages of mitosis, and the key events of each:
| Stage | Key Events |
|---|---|
| Prophase |
– DNA supercoils and condense into chromosomes – Nuclear envelope breaks down – Centriole divides, and the daughter centrioles migrate to opposite poles – Spindle begins to form from centrioles (in animals) – Spindle begins to form from cytoplasm (in plants) |
| Metaphase |
– Chromosomes line up along the equator (also called the metaphase plate) – Spindle fibres attach to the chromosomes’ centromeres – Restriction point (M phase) ensures all chromosomes are correctly attached |
| Anaphase |
– Chromosome centromeres divide, separating sister chromatids – Spindle fibres shorten, pulling chromatids to opposite sides of the cell – Motor proteins pull sister chromatids towards opposite poles |
| Telophase |
– Chromatids reach the poles – Chromosomes uncoil and uncondense (back into chromatin) – Nuclear envelopes reform around each set of chromosomes – Spindle fibres disassemble |
The diagram below shows the changes which occur in each phase of mitosis:
Cytokinesis
Cytokinesis is the division of the cellular cytoplasm (to produce two genetically identical cells).
In animal cells the plasma membrane ‘pinches inwards’ around the equator, contracting further until the two halves separate.
In plant cells, the cell elongates and then divides itself into two by forming a new cellulose cell wall along the metaphase plate and depositing a new plasma membrane on either side of it.
Importance of Mitosis
Mitosis ensures that DNA is replicated exactly to ensure it remains the same within all cells of an organism, and in any asexually produced offspring.
It is essential in the life cycles of multicellular eukaryotes:
- Growth: Of either the whole organism, or tissues and organs.
- Tissue repair: By replacing damaged or dead cells
- Asexual reproduction: For some animals, plants and fungi.