Vinita P.

Every somatic (body) cell undergoes 2 distinct phases during its life cycle, the 'resting' phase or interphase and the cell division phase i.e. the Mitotic phase.

The interphase or 'resting' phase is where the cell grows from a newly formed relatively small daughter cell to a mature cell ready for cell division like it's mother cell. It's called 'resting' phase because distinct chromosomes are not visible in this stage. However it has 3 sub phases taking place here - G1, S and G2

1.G1 phase- The cell volume increases as more RNA, proteins, mitochondria and chloroplasts (in plant cells only) are synthesised in the cytoplasm. Then the cell either temporarily withdraws from the cell cycle or goes into the next phase to prepare for cell division.

2. S  phase - The cell synthesises more DNA and the chromosome duplicates into 2 sister chromatids that are attached at the centromere. This helps to maintain the chromosome number once cell division is over.

3. G2 phase-  New RNA and proteins necessary for cell division are synthesised and now the daughter cell is mature enough and ready to enter the m-phase for cell division.

MITOTIC / M-Phase has 2 steps-

1. Karyokinesis (nuclear division) and 2. Cytokinesis (Cytoplasmic divsion)

Karyokinesis is carried out in 4 phases- Prophase, Metaphase, Anaphase, Telophase

PROPHASE- 

i) Centrioles outside the nucleus duplicates and start moving towards the opposite poles of the cell

ii)Within the nucleus, each duplicated chromosome becomes short and thick  

iii) Spindle fibres are formed between the centrioles at the poles of the cell

iv) Nuclear membrane and nucleolus disappear

METAPHASE-

Duplicated Chromosomes line up in the plane of the equator and get attached to the spindle fibres at their centromere.

ANAPHASE-

i) The Centromere attached to the 2 sister chromatids of each chromosome splits 

ii) THe sister chromatids are drawn towards the opposite poles due to the shortening of the spindle fibres

TELOPHASE

i) Sister Chromatids reach the opposite poles

ii) Chromatids thin out to form chromatin fibres

iii) Spindle fibres Disappear

iv) Nuclear memebrane and nucleolus reappear around each set of chromatin fibres at the poles forming 2 distinct nuclei.

vi)  A furrow/cleavage appears in the equatorial plane of the cell membrane in an animal cell while a cell plate starts appearing from the centre towards the periphery in the plant cell.

This completes the process of Karyokinesis as now 2 nuclei having the same number of chromtin/chromosomes as in the mother cell have now been formed.

Now comes the last step of mitotic cell division, CYTOKINESIS, the division of the cytoplasm. Here the clevagedeepens further in an animal cell while the cell plate grows further towards the periphery ina plant cell. This results in their seperation into 2 identical daughter cells.

Thus, the mother cell has completed its life cycle through the INTERPHASE ( G1, S and G2 phases) and M-PHASE(1. Karyokinesis- Prophase, Metaphase, Anaphase and Telophase  and 2. Cytokinesis) resulting in 2 identical daughter cells with the same number of chromosomes