#92 Mitosis (English)

Mitosis is the division of the nucleus into two genetically identical daughter nuclei. In other words, most of our cells (except sex cells) have the ability to clone themselves, a process that is very much central to our lives.  In summary mitosis is required for 4 functions:

1.      Growth: multicellular organisms increase their size by increasing their number of cells through mitosis.

2.      Asexual Reproduction: several eukaryotic organisms reproduce asexually through mitosis.

3.      Embryonic Development: in order to develop, a zygote (a fertilised egg) will undergo mitosis and differentiation in order to develop into an embryo.

4.      Tissue repair: damaged mitosis can recover damaged tissue by replacing damaged/dead cells.

 

The cell cycle is divided into phases. Interphase is where the cells spend the majority of their time.  It consists of the parts of the cell cycle that don’t involve cell division. 

 

The cell’s preparation for mitosis, known as Interphase, is further divided into five stages:

·         G0 – resting stage of the cell. (not included in this diagram)

·         G1 (Gap 1) – the cell increases the volume of the cytoplasm, produces organelles and synthesises proteins.

·         S (Synthesis)  - the DNA is replicated.

·         G2 (Gap 2) – the cell essentially does the same as in G1. 

The mitotic phase is much more complex than those previously mentioned, and consists of 4 different stages.

1.      Prophase:

-        Centrosomes move to opposite poles of the cell and spindle fibres begin to form between them.

-        DNA supercoils (chromatins condense with the help of specific proteins called histones) and become what we call ‘sister chromatids.’

-        The nuclear membrane breaks down. 

2.      Metaphase:

-        Spindle fibres (made up of microtubules) from each of the two centrosomes attach to the centromere of each pair of sister chromatids.

-        Contraction of the spindle fibres cause the sister chromatids to line up along the centre of the cell.

3.      Anaphase:

-        Continued contractions of the microtubule spindle fibres cause the separation of the sister chromatids (thus are now referred to as chromosomes.

-        Chromosomes move to the opposite poles of the cell. 

4.      Telophase:

-        Chromosomes now arrive at the poles and uncoil (de-condense) to chromatin.

-        Microtubule spindle fibres disappear

-     New nuclear membranes reform around each set of chromosomes.

 

SAG