Eukaryotic cell division

The cell cycle

The life of a cell from one division to the next is called the cell cycle, which applies to eukaryotic cells only. The cell cycle has two main phases: Interphase and the mitotic phase. Interphase is when the cell grows and performs its normal functions. Typically, 90% of the cell cycle is spent in interphase. Interphase can be further divided into:

• Growth 1 (G1), where the cell returns to its original size and replicates its organelles.

• Synthesis (S) phase, where DNA is replicated in preparation for mitosis.

• Growth 2 (G2), where the cell prepares for mitosis.

During interphase, no chromosomes are visible as the DNA is present as chromatin.

The mitotic (M) phase is where the cell divides to form two daughter cells. 

The cell cycle

The stages of mitosis are as follows:

• During prophase, the chromosomes condense and become visible, the nuclear envelope breaks down, while the centrioles move to opposite poles of the cell and the mitotic spindle fibres form.

• During metaphase, the nuclear envelope breaks down, and the chromosomes align along the equator of the cell. Spindle fibres connect the centrioles to the centromeres of the chromosomes.

• During anaphase, the centromeres split, allowing the chromatids to separate. The chromatids are pulled toward the opposite poles by the spindle fibres.

• During telophase, the spindle fibres disperse, and nuclear envelopes reform around the chromosomes. The chromatids uncoil and return to chromatin, becoming too thin to see.

• Finally, during cytokinesis, the cell splits into two.

The stages of mitosis

Nomenclature of chromatids, chromosomes and centromeres

Regulation of the cell cycle

The cell cycle is usually tightly regulated, with checkpoints at the beginning of each stage, where control genes determine if a cell can proceed to the next phase. However, these control genes can sometimes mutate due to carcinogenic agents such as viruses, ionising radiation, chemicals, or other environmental factors. When these genes are mutated, they fail to regulate the checkpoints properly, causing cells to divide uncontrollably, leading to the formation of a tumour (a mass of identical cells) and potentially cancer, if tumour cells spread and form new tumours elsewhere in the body.

We can use our understanding of the cell cycle and its regulation to develop anti-cancer drugs, which typically block the cell cycle to compensate for defective checkpoints. For example:

• Adriamycin inhibits DNA helicase, stopping DNA replication.

• Methotrexate inhibits nucleotide synthesis, also stopping DNA replication.

• Taxol inhibits the formation of the mitotic spindle, preventing mitosis.

Definitions