6.5 The Cell Cycle, Mitosis and Cancer

Prokaryotes reproduce by binary fission: DNA replicates, copies segregate as the cell elongates, and membrane plus wall divide the cytoplasm. In eukaryotes, the cell cycle contains interphase, mitosis and cytokinesis. G₁ supports growth and normal gene expression, S phase replicates DNA, and G₂ completes preparation. Interphase is metabolically active and usually occupies far more time than visible nuclear division.

The G₁ checkpoint assesses size, nutrients, growth signals and DNA damage. The G₂ checkpoint tests whether replication is complete and DNA is intact. The spindle checkpoint prevents anaphase until every chromosome is correctly attached. Cyclins rise and fall during the cycle; binding activates cyclin-dependent kinases, which phosphorylate target proteins and drive particular transitions.

Proto-oncogenes normally promote division or survival in appropriate contexts. Gain-of-function mutation or amplification can turn one into an overactive oncogene. Tumour suppressor genes restrain division, repair DNA or promote apoptosis; loss of their function removes a brake. Cancer commonly requires accumulated changes affecting several control systems rather than one mutation acting alone.

A benign tumour remains localized; a malignant tumour invades surrounding tissue. Cells that detach, travel and establish secondary tumours undergo metastasis. Apoptosis packages a damaged cell into membrane-bound fragments for phagocytosis, limiting inflammation and preventing inheritance of irreparable DNA. Cancer cells often evade apoptosis as well as divide inappropriately.

The mitotic index is the fraction of observed cells in mitosis. A high value can indicate rapid proliferation and helps compare tumour grade or growing tissues, but it is also influenced by the duration of mitosis. The calculation requires a representative sample and clear criteria for identifying condensed chromosomes.