The cell cycle is the series of events that a cell goes through as it grows and divides. It consists of four main phases:

1. G1 Phase (Gap 1)

This is the first phase after a cell is produced by cell division.

During G1, the cell grows in size, synthesizes proteins, and produces organelles.

The cell is metabolically active and prepares the necessary materials for DNA replication.

There is a checkpoint towards the end of G1 to determine if the cell is ready for DNA synthesis.


2. S Phase (Synthesis)

During this phase, DNA replication occurs.

Each chromosome is duplicated, resulting in two identical sister chromatids held together by a centromere.

By the end of the S phase, the cell has double the amount of DNA, though the chromosome number remains the same.


3. G2 Phase (Gap 2)

This phase follows DNA synthesis and is a period of further growth and preparation for mitosis.

The cell continues to grow and produces proteins, particularly those required for chromosome segregation and cell division.

There is another checkpoint at the end of G2 to ensure DNA replication has occurred correctly and the cell is ready to divide.


4. M Phase (Mitosis)

This is the phase where the cell divides into two daughter cells.

It consists of two main processes:

Mitosis: Division of the cell's nucleus into two genetically identical nuclei. Mitosis is further subdivided into four stages: prophase, metaphase, anaphase, and telophase.

Cytokinesis: Division of the cell's cytoplasm, resulting in two separate cells.



Checkpoints in the Cell Cycle

G1 Checkpoint: Determines if the cell is of adequate size, has enough energy, and has no DNA damage before proceeding to DNA synthesis.

G2 Checkpoint: Ensures that DNA replication has been completed successfully and checks for DNA damage before mitosis.

M Checkpoint (Spindle Checkpoint): Ensures that all chromosomes are correctly attached to the spindle apparatus before cell division proceeds.


Cells that do not pass these checkpoints may enter a non-dividing state called G0 phase, where they exit the cell cycle and perform their specialized functions.

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