The document outlines the cell cycle's stages, including G1, S, G2, and M phases, detailing how cells grow, replicate DNA, and divide into two identical cells. It emphasizes the regulation of the cell cycle through checkpoints that prevent damaged DNA from being replicated, involving proteins like p53 and retinoblastoma (Rb) that control cell progression. Additionally, it discusses the quiescent state, G0, where some cells exit the cycle but can re-enter it under certain conditions.

1. Rajkumari Lodhi
Assistant Professor
Department of Pharmacology
Bhopal Madhya Pradesh
CELL CYCLE AND ITS
REGULATION

2. cell division :
Animal development from a single-cell zygote to fertile adult requires many rounds of
cell division.
During each division, cells complete an ordered series of events that collectively form
the "cell cycle".
This cycle includes accurate duplication of the genome during the DNA synthesis
phase (S phase), and segregation of complete sets of chromosomes to each of the
daughter cells in M phase (Figure 1A).
The somatic cell cycle also contains "Gap" phases, known as G1 , which connects the
completion of M phase to initiation of S phase in the next cycle, and G2 , which
separates the S and M phases.

3. cell cycle
The cell cycle is the process a cell undertakes to replicate all of its genetic material and
divide into two identical cells. There is different stages of the cell cycle and what happens
in each stage. We will also consider the regulation of the cell cycle, and look at some
examples of its dysregulation.
Phases of the Cell Cycle
The cell cycle is a 4-stage process consisting of
1. Gap 1 (G1),
2. synthesis (S),
3. Gap 2 (G2) and
4. Mitosis (M),
which a cell undergoes as it grows and divides. After completing the cycle, the cell either
starts the process again from G1 or exits the cycle through G0. From G0, the cell can
undergo terminal differentiation.
The stages in the cell cycle between one mitosis and the next, which include G1, S and
G2, are known collectively as the interphase.

4. G1 phase
•Cell increases in size
•Cellular contents are duplicated
S phase
•DNA replication
•Each of the 46 chromosomes (23 pairs) is replicated by the cell
G2 phase
•Cell grows more
•Organelles and proteins develop in preparation for cell division
M phase
•Mitosis followed by cytokinesis (cell separation)
•Formation of two identical daughter cells
G0 phase
While some cells are constantly dividing, some cell types are quiescent. These cells exit G1
and enter a resting state called G0. In G0, a cell is performing its function without actively
preparing to divide. G0 is a permanent state for some cells, while others may re-start division if
they get the right signals.

6. Regulation
 The progression of cells through the cell cycle is controlled by various checkpoints at different stages.
These detect if a cell contains damaged DNA and ensure those cells do not replicate and divide.
 The restriction point (R) is located at G1 and is a key checkpoint.
 The vast majority of cells that pass through the R point will end up completing the entire cell cycle.
 Other checkpoints are located at the transitions between G1 and S, and G2 and M.
 If damaged DNA is detected at any checkpoint, activation of the checkpoint results in
increased p53 protein production. p53 is a tumour suppressor gene that stops progression of the cell cycle
and starts repair mechanisms for the damaged DNA.
 If this DNA cannot be repaired, it ensures the cell undergoes apoptosis and can no longer replicate.
 This cell cycle is also closely regulated by cyclins which control cell progression by activating cyclin-
dependent kinase (CDK) enzymes.

7. An example of a tumour suppressor protein is retinoblastoma protein (Rb). Rb restricts
the ability of a cell to progress from G1 to S phase in the cell cycle. CDK
phosphorylates Rb to pRb, making it unable to restrict cell proliferation, thereby
inhibiting its cell growth-suppressing properties. This allows cells to divide normally in
the cell cycle.

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