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Mitosis is the process of nuclear division that produces two identical daughter cells with a diploid complement of chromosomes from the parent cell. Cytokinesis usually occurs with mitosis, this is the process where the cell itself divides in order to yield two daughter cells. Mitosis only occurs in somatic tissues which are ordinary body cells, such as, liver cells, skin cells, etc[1]. In multicellular eukaryotic cells mitosis is a way for cells to repairing tissues, to accommodate for growing organisms, embryonic development, production of reticulocytes and to form identical cells to replace dying cells. In unicellular eukaryotic organisms, for example, yeast, mitosis is a way to reproduce asexually[2]. Check it out on The Mitosis Cycle on YouTube. It can be broken down into four main stages seen in animal cells:



If a cell is not undergoing mitotic cell division, the cell is in interphase. In this phase, the chromosomes are invisible through a light microscope[3].

Interphase constitutes for most of the cell cycle;

It is the preparation period preceding mitosis, in which the cell grows and it's DNA is replicated. Interphase can be divided into 3 main phases;

G1( gap phase 1 ), S ( synthesis phase ) and G2 ( gap phase 2).

During Gap phase 1 biosynthetic activity in the cell is increased as the cell grows and replicates it's organelles whilst producing spare proteins.

During S phase the cell must replicate its DNA, in order for the two daughter cells produced via the following division to have a complete set of genetic material. Chromosomes are duplicated during this same phase, this produces 2 sister chromatids, allowing each daughter cell which results from the mitotic division to have the diploid number of chromosomes.

During Gap phase 2 the cell continues to grow and it produces specific proteins necessary for cell division so that mitosis can follow interphase.

Another vital event which occurs during interphase is that the cell repairs any DNA damage, to ensure that no undesirable mutations are passed on to the progeny cells[4].


During interphase, the chromosomes are already doubled, this has two closely associated subunits called chromatids. Each pair of chromatids is a product from duplication of one chromosome in the S phase from interphase. These chromatids are held together by the centromere[5]. Throughout the process of prophase the chromosomes condense meaning they get shorten and thicken to form visibly distinct threads within the nucleus. This is brought about by the action of large proteins called condensins. Each </span>chromosome at this point consists of two </span>chromatids held together at the </span>centromere (central point). Meanwhile, outside the nucleus, the mitotic spindles assemble this is between the two centrioles. The centrioles have replicated and moved apart in order to do this.


The beginning of prometaphase includes the breakdown of the nuclear envelope, this, therefore, means the spindle microtubules from the centrioles can attach to the centromere of the chromosomes. This is by kinetochores which then undergo active movement. The centrioles are at opposite poles when this stage begins. During this phase chromosomes are recognisable as double structures[6].

The four phases of mitosis.


The mitotic spindle consisting of microtubules formed by the polymerization of tubulin begins to form. There are three types of microtubule in the spindle:

  1. Anchor (centrosome to cell membrane)
  2. Arch (between centrosomes)
  3. Attachment ( to chromosomes)

NB. Centrosomes are located at the ends/poles of the spindle and are microtubule organising centres where tubulin polymerisation is initiated. These Centrosomes are formed by duplication of one centrosome during interphase.

The microtubules form when new tubulin subunits are added to the growing end. However, this can become unstable and the process of depolymerization can be initiated causing the polymer to shrink. This process is regulated by a number of proteins that maintain a balance between polymerization and depolymerization[7].

The next step in metaphase is the formation of the kinetochore, a site of attachment between the chromosome and the spindle microtubules[8]. Only chromosomal microtubules that make contact with the kinetochore however, will become stabilised and others will depolymerise.

The final part involves a position near to the centre of the cell, known as the metaphase plate. The kinetochore signals for each chromosome to move to the plate via dephosphorylation of certain proteins. The cell can then only move on to anaphase when all kinetochores are under tension and are aligned on the metaphase plate.


Proteins holding the chromatids together dissolve and centromeres separate (This is also known as disjunction).There are two stages to anaphase. Anaphase A, describes the way the chromosomes move to opposite poles of the cell, the Kinetochore microtubules begin to get shorter and shorter alongside. At anaphase B, the spindle fibres also get shorter and therefore move apart[9] it happens after the chromatids are no longer joined and the daughter chromosomes have moved some distance apart.[10] Spindle fibres are signalled by the cell to lengthen causing the identical sister chromatids of each chromosome to be pulled toward opposite poles of the spindle. This occurs by the centromeres splitting and getting pulled to the opposite poles of the cell towards the centrioles. These processes contribute to chromosome segregation. The two groups that now lie in each end of the spindle contain the same number of chromosomes that were present in the original interphase nucleus[11].

Above shows the four stages of mitosis[12].


In this final stage of mitosis, a nuclear envelope forms around each group of chromosomes and cytokinesis occurs, where the cytoplasm divides into two. The spindle completely disappears and chromosomes undergo the reversal of condensation, deeming them no longer visible under the light microscope. At the end of telophase, the two separate daughter cells assume interphase appearance with fully developed nucleoli.


The two individual nuclear envelopes are now formed again with the same amount of chromosomes in each. The cytoplasm now splits to form the two cells by a contractile ring of actin and myosin filaments, this 'pinches' the cell in two forming the two identical daughter cells[13].


  1. Hartl, D. Ruvolo, M. (2012) Genetics. Burlington: Jones and; Bartlett learning. pg 119, 120
  2. Hartl, D. Ruvolo, M. (2012) Genetics. Burlington: Jones and; Bartlett learning. ph 119, 120
  3. Hartl, D. Ruvolo, M. (2012) Genetics. Burlington: Jones and Bartlett learning. pg 119, 120
  4. AS-Level Biology-The revision guide.Cumbria: Coordination group publications Ltd. 2. Hartl, D.L. and Ruvolo, M,2011.Genetics-analysis of genes and genomes.8th ed.Burlington MA: Jones and Bartlett Learning.
  5. Johnson, A. et al. (2008) The Cell. 5th edn. Abingdon: Garlan Science. pg 1071, 1072, 1073.
  6. Johnson, A. et al. (2008) The Cell. 5th edn. Abingdon: Garlan Science. pg 1071, 1072, 1073.
  7. Hartl, D. Ruvolo, M. (2012)Genetic: analysis of Genes and Genomes 8th edition jones and bartlett learning USA
  9. Johnson, A. et al. (2008) The Cell. 5th edn. Abingdon: Garlan Science. pg 1071, 1072, 1073.
  10. Alberts, B. et al. (2015) Molecular Biology of the Cell. 6th edn. Garland Science. pg 994
  11. Johnson, A. et al. (2008) The Cell. 5th edn. Abingdon: Garlan Science. pg 1071, 1072, 1073.
  13. Johnson, A. et al. (2008) The Cell. 5th edn. Abingdon: Garlan Science. pg 1071, 1072, 1073.
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