The Stages 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)
- Although mitosis is, in reality, one continuous process, it can be divided into four main stages
- 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 the sake of 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 (which consist of two centrioles in animal cells)
- The nuclear envelope (nuclear membrane) breaks down into small vesicles
Prophase stage of mitosis where chromosomes condense into visible structures
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
- Each sister chromatid is attached to a spindle fibre originating from opposite poles
Metaphase, where chromosomes line up along the equator of the cell
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, where chromosomes are pulled to the poles of the cell
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
Telophase, where the nuclei reform and the cell begins to split into two
Examiner Tip
Make sure you learn the four stages of mitosis. Cytokinesis is often mistaken as a stage of mitosis but remember this is a separate part of the cell cycle.
The acronym PMAT can be helpful to remind you what happens during each stage of mitosis:
P = Prophase, where the cell Prepares to divides
M = Metaphase, where the chromosomes align along the Middle
A = Anaphase, where the chromosomes move Away from each other
T = Telophase, where Two nuclei reform
The chromosome number is important too; after interphase but before the parent cell undergoes mitosis, the human parent cell nucleus actually contains 92 DNA molecules! This is because during interphase (S phase), the 46 DNA molecules in the parent cell have replicated to form sister chromatids. As human cells have a diploid number of 46 this replication results in 92 molecules. This ensures the two daughter cells will be diploid (have 46 chromosomes each) when mitosis occurs. Remember to read the questions carefully as only human diploid cells have 46 chromosomes so if the question refers to another organism, its diploid number will be different.
