Cell Division

Cell cycle:

It is the alternation between mitosis and interphase.
It is the cyclical sequence of events involving DNA replication and cell division (mitosis) to produce two identical daughter cells.
Basically, it is done by duplication of cellular contents (interphase), nuclear division (mitosis) and cytoplasmic division (cytokinesis).
Events/Phases of cell cycle:
1) Interphase
2) Mitosis phase (division of nucleus)
Also known as the preparatory phase for cell division.
Consists of 3 phases
• G1 phase
• S phase
• G2 phase
G1 phase (Pre synthetic phase)
- It is the preparatory phase for replication of DNA
- Here, transcription (RNA synthesis), translation (protein synthesis).
- Act as 1st checkpoint:
If any damage has occurred in DNA, then the G1 phase
does not go to S-phase for further process.
First DNA would repair, then only proceed forward.

S-phase (DNA synthesis phase)
Here, replication (synthesis) of DNA occurs.
From one DNA another same DNA is synthesized resulting in two identical DNA.
G2 phase (Pre-mitotic phase):
- Preparatory phase for mitosis with further protein & organelles synthesis occur need for mitosis
- Act as 2nd checkpoint:
If any error has occurred in replication, then the G2 phase is not proceeding forward to the mitosis phase.
Mitotic phase (M phase):
- Here nuclear division occurs and cells divide into two daughter cells.
- After passing all checkpoints (G1, G2) phase with no error, the mitotic phase occurs.
• But it is absent in neurons (Neuron cannot regenerate).


Go phase: 
Some cells like neurons & muscle cells may stop to undergo mitosis permanently after the last mitotic event these cells do not enter the cell cycle & are said to be in the resting stage, which is known as the Go phase.
E.g. Neuron, Mature T-lymphocyte.
Due to the abnormal function of the spindle fibers, one or more chromosomes fail to migrate properly during anaphase.
There will be an unequal division of chromosome number in the divided two daughter cells.
This abnormality of the chromosome is known as non-dysfunction.
In this case, one daughter cell receives extra chromosomes & the other daughter cell has less no. of chromosomes.
This leads to monosomy, trisomy.