Stages and cellular activities during the cell cycle and divisions

The cell cycle describes the events that take place as one parent cell divides into two new daughter cells.
It consists of two broad stages: interphase and the mitotic (M) phase.
Interphase is the period of cell growth and DNA replication, while the M phase includes mitosis (cell division) and cytokinesis (cytoplasm division).

Interphase consists of three phases: G1 (first gap), S (synthesis), and G2 (second gap).
During the G1 phase, the cell grows in size, synthesises new proteins and organelles, and carries out its specific functions. The cell prepares for DNA replication.
The following S phase is when DNA is replicated, thus each chromosome now consists of two sister chromatids.
In the G2 phase, the cell continues to grow and prepare for mitosis. It produces proteins and organelles and begins to reorganise its components in preparation for mitotic division.

The mitotic (M) phase includes both mitosis and cytokinesis.
Mitosis is broken down into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.
Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down, and the mitotic spindle forms.
Prometaphase: The chromosomes continue to condense, kinetochores appear at the centromeres, and spindle fibres attach to the kinetochores.
Metaphase: The chromosomes line up along the middle of the cell, known as the metaphase plate.
Anaphase: The sister chromatids separate and are drawn towards opposite poles of the cell by the spindle fibres.
Telophase: Chromosomes decondense, a new nuclear envelope forms around each set of chromosomes, and the mitotic spindle breaks down.
Cytokinesis then occurs, dividing the cytoplasm to create two independent daughter cells, each with a complete set of chromosomes and fully functional cellular components.

The cell cycle is regulated by a series of signalling pathways to ensure correct division and to prevent uncontrolled cell growth.
Checkpoints occur at specific points within the cycle to ensure each stage has been completed correctly before the cell progresses to the next stage.
Protein complexes of cyclins and cyclin-dependent kinases (CDKs) play crucial roles in controlling cell cycle progression.
p53, nicknamed the “guardian angel of the genome”, is a crucial regulator that can trigger cell cycle arrest or apoptosis (cell death) in response to DNA damage or other cellular stress.
Disruptions in cell cycle control, such as mutations in genes coding for key regulators, can lead to uncontrolled cell growth and cancer.