Cell division

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Cell division is a process which occurs in all living cells. There are two types of cell division which are mitosis and meiosis. Mitosis, cell nuclear division in which the parental cell divides into two genetically identical daughter cells with the purpose usually being for growth and repair. Meiosis, cell nuclear division where the parental cell divides into four genetically distinct haploid cells which are known as gametes.

Mitosis consists for four stages:

  1. prophase
  2. metaphase
  3. anaphase
  4. telophase

During prophase the chromosomes condense and become visible, while in the cytplasm of the cell spindles form. The chromosomes align at the centre of the cell during metaphase. During anaphase chromosomes move towards opposite poles of the cell. Telophase is the reverse of prophase[1].


Meiosis which undergoes 2 segregation is subdivided into:

       1. Meiosis I

       2. Meiosis II

Meiosis I consists of 4 stages which are Prophase I, Anaphase I, Metaphase I and Telophase I. Meiosis II consists of 4 stages as well which are Prophase II, Anaphase II, Metaphase II and Telophase II.

For Meiosis I, Prophase I is the longest phase in Meiosis. During prophase I a cell will undergo 5 stages which are leptotene, zygotene, pachytene, diplotene and diakinesis. At leptotene, the homologous chromosome start to condense. At zygotene, synaptonemal complex is formed. At pachytene, the synapsis process is completed and the homologous chromosomes start the crossing over. This stage can stay for days until the desynapsis occur. At diplotene stage, desynapsis occur causing the disappearance of synaotonemal complex and the chiasma is now visible. At diakinesis stage, the bivalent is ready for the segregation.[2] At the end of prophase I, the nuclear membrane disappears. At Metaphase I, the homologous chromosome aligns on the metaphase plate which is the middle of the cell. At Anaphase I, the contraction of the  spindle fibre causes one of the homologous chromosome move to the opposite pole for disjunction. At Telophase I, the nuclear membranes reappear and form around the dyads. Cytokinesis I occurs.

After Meiosis I, the cells will undergo interphase II before they proceed to Meiosis II. However, in Interphase II, only G1 and G2 phase will be involved but not S phase. Therefore, there is no further DNA synthesis in Meiosis II. As for Meiosis II, during Prophase II of Meiosis II, there will not be crossing over occur nor the formation of chiasmata. The dyad consists of a pair of sister chromotids. At this phase, the nuclear membrane disappears and the centriole duplicates. At Metaphase II, the sister chromatids align on the metaphase plate. At Anaphase II, the contraction of spindle fibre causes one of the sister chromatids move to the opposite pole for segregation. At Telophase II, the nuclear membrane reappears and cytokinesis II occurs and 4 haploid daughter cells which are genetically distinct produced. [3]


References

  1. University of illinois(n.d)cell divison'Mitosis and Meiosis. Available at:Www.uic.edu/classes/bios/bios100/lecturesf04/am/lect16.htm. Last accessed 4.12.15
  2. Alberts B., Johnson A., Lewis J., Morgan D., Raff M., Roberts K. and Walters P.. Molecular Biology of The Cell 8th ed. 2015: 1004- 1010
  3. Vidyasagar A.. What is Meiosis?. 2018. Available from: https://www.livescience.com/52489-meiosis.html
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