The document summarizes key aspects of the cell cycle. It describes that the cell cycle consists of interphase and the division phase. Interphase includes G1, S, and G2 phases where the cell grows and duplicates its DNA in preparation for division. The division phase is called M-phase where the nucleus and cytoplasm divide. There are three main checkpoints in the cell cycle - G1, G2, and M-phase checkpoints - that ensure proper conditions for progression and division by assessing the cell's preparation and DNA status. The duration of the cell cycle varies depending on cell type and external factors.

1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai
Cell cycle
Dr. Sonia Bajaj
(Head of Department)

2. Cell cycle
Cell division is a very important process in all living organism.
Cell division was first studied by Prevost and Dumas when they described cleavage in zygote of frog.
During the division of cell, DNA replication and cell growth also take place.
Although cell growth (in term of cytoplasmic increase) in a continuous process, DNA synthesis occurs only during
one specific stage in the cell cycle.
The replicated chromosome DNA are then distributed to daughter nuclei by a complex series of event during cell
division. These event are themselves under genetic control.
Complete life cycle of a cell is called as cell cycle.
Phases of cell cycle
A typical eukaryotic cell cycle is illustrated by human cell in culture. These cell divides once in approximately
every 24 hours.
Yeast can progress through the cell cycle in only about 90 minutes.
The time period of cell cycle is varied from organism to organism and also from cell type to the cell type.

3. cell cycle involved two Part
1. Interphase
2. Division phase / M- phase
1. Interphase- This is phase between end of one cell division and
beginning of the next.
In interphase cell growth in size and prepare itself for next division.
Interphase is most active phase of cell cycle.
The Interphase laste more than 95% of the duration of cell cycle.
In interphase metabolism of cell increase.

4. Interphase divided into three sub stages
G1-phase / (1st Gap phase)-
It is also Known as pre DNA synthesis phase .
It is characterized by increase in size of cell.
This comes after telophase of earlier division .
in this stage synthesis of RNA and protein take place.
chromosome seem to be in the form of chromatin network in the
nucleus. 10 h. duration.
S phase /DNA synthetic phase –
Replication of nuclear DNA and synthesis of histone protein take place
in s- phase.
Replication of cytoplasmic DNA may occur in any stage of cell cycle.
In animal cell during the S phase, DNA replication beings in the
nucleus, and the centriole duplicates in the cytoplasm.
Centriole splits in late s- phase.
8 h. duration.

5. G2 phase /(2nd Gap phase) / (Pre mitosis phase)
It is also known as post DNA synthesis phase.
Actual preparation (Final preparation) of M-phase occurs during this phase.
During this phase proteins are synthesized in preparation for mitosis while
cell growth continues.
After G2 to phase cell enters in division or M-phase.
Mitochondria ,plastids also duplicate during this phase.
5 h. duration.
G0 phase /Resting state-
some cell in the adult animal do not appear to exhibit division (eg. Heart
cells) and many other cells divide only occasionally, as needed to replace cells
that have been lots because of injury or cell death.
These cells that do not divide further exit G1 phase to enter an
inactive stage called quiescent stage (G0) of the cell cycle.

6. 2. Division phase/M-Phase-
It is also called Mitotic phase .
Division phase lasts for only about an hour in the 24hour duration of cell
cycle of a human cell.
It is the phase of shortest time in cell cycle.
Only 1h.
It consists of 2 phase-
1. Karyokinesis –division of nucleus, which is completed in 4 steps-
Prophase
Metaphase
Anaphase
Telophase
2. Cytokinesis- division of cytoplasm
After M-phase the cell may go to interphase or may go into G0 phase.
Duration of cell cycle depends on the cell type and external
factors like Temperature/Oxygen/Food etc.

7. CELL CYCLE CHECK POINTS
Cell cycle check points are control mechanism in Eukaryotic
cells which ensure proper division of cells.
Each checkpoint serves as a potential point along the cell cycle
during which, condition and the progress of the cell is
assessed.
Eukaryotic cell have a cell cycle control system, which involve a
regulatory protein belonging to the family of protein kinases
also known as Cyclin-Dependent kinases. (CDKS).
CDKS bind to regulator protein known as cyclins to form a
specific cyclin CDK Complex, which get activated during a
particular phase of cell cycle.
There are three checkpoint-
1. G1 check point
2. G2 check point
3. M check point.

8. G1 check point
This checkpoint is at G1/ S transition.
G1 checkpoint is also known as Restriction point in mammalian cells and start
point in yeast. This is point at which the cell becomes committed to enter the
cell cycle.
Once the cell phase the G1 checkpoint, it become irreversibly committed to
divide.
As the cell progresses growth G1 phase, it can either delay G1 or enter or
proceed past the restriction point.
The decision to enter a new round of cell division occurs when the cell activates
cyclin cdk dependent transcription.
So during G1 checkpoint, the cell is assessed for the following preparation in
order to enter the next phase of cell cycle.
DNA damage
cell Size
Does the cell have enough ATP protein enzymes synthesized?

9. G2 check point
This checkpoint is at the G2/M transition
During this checkpoint, and cell is checked for
DNA damage or incomplete replication
Protein synthesis and accumulation which give a positive feedback and
help the cell of progress to next phase.
Metaphase /M checkpoint
Checkpoint is at the transition from metaphase to anaphase.
It is known as mitotic spindle checkpoint.
During the checkpoint, the cell is checked for
Alignment of chromosome at the mitotic phase.
Tension create by bipolar attachment is sensed.
Hear anaphase promoting Complex get activated.
If the mechanism to detect and damage in the DNA fails it
results is cancer.

10. References
Genetics by PS Verma
Jantu Vigyan- S.M. sexsena
Jantu Vigyan- Dr.H.N. Baijal