Eukaryotic Cell Cycle and Division

The Eukaryotic Cell Cycle

The eukaryotic cell cycle consists of four main phases: G1, S, G2 and M.
During the G1 phase, cells grow, produce RNA and protein, and prepare for DNA replication.
The S phase sees the replication of DNA, resulting in each chromosome having two sister chromatids.
In the G2 phase, cell growth continues and proteins necessary for cell division are made.
The M phase, or the Mitosis phase, is when the cell divides. It has five stages: prophase, prometaphase, metaphase, anaphase and telophase.
Cytokinesis occurs after mitosis, dividing the cytoplasm and resulting in two daughter cells.

The Stages of Mitosis

Prophase marks the start of mitosis where chromosomes condense and the nuclear envelope breaks down.
During prometaphase, spindle fibres from centrioles attach to the kinetochore of chromosomes.
Metaphase is characterised by chromosomes aligning on the metaphase plane.
In anaphase, sister chromatids separate and move towards opposite centrioles.
Telophase sees separated chromatids reach opposite poles, the spindle fibres disperse, nuclear envelope reform and chromosomes decondense.

Role of Control Proteins

Cyclin-dependent kinases (CDKs) and cyclins are two classes of control proteins the regulate the cell cycle.
Specific cyclins bind to their specific CDKs, activating the CDKs to phosphorylate target proteins, which advances the cell cycle.
Cyclin levels fluctuates while CDK levels remain constant.
Abnormalities in cell cycle control can lead to uncontrolled cell division - a phenomenon seen in cancers.

Cell Cycle Checkpoints

There are several checkpoints in the cell cycle that monitor and regulate progress: the G1 checkpoint, G2 checkpoint, and M checkpoint.
The G1 checkpoint checks for cell size, nutrients, growth factors and DNA damage.
The G2 checkpoint ensures all chromosomes have been replicated and that replicated DNA is undamaged.
The M checkpoint checks for chromosome attachment to the spindle (metaphase check).

Meiosis: A Form of Cell Division

Unlike mitosis, meiosis results in four daughter cells each with half the number of chromosomes of the parent cell.
Meiosis constitutes two successive divisions: Meiosis I (reductional division) and Meiosis II (equational division).
Meiosis increases genetic diversity through crossing over and independent assortment.
Faults in meiosis, such as nondisjunction, can lead to genetic disorders like Down syndrome.