All cells arise from other cells: Mitosis is a controlled process.

The idea

Mitosis is controlled so that cells divide only when appropriate. The cell cycle is regulated at checkpoints: at the end of G1 the cell checks it is large enough and the DNA is undamaged before committing to replication; at the end of G2 it checks DNA replication is complete and the DNA is undamaged before mitosis; during metaphase it checks every chromosome is correctly attached to the spindle before chromatids separate. Genes control this. Some genes code for proteins that stimulate the cycle to continue; others code for proteins that halt the cycle so damaged DNA can be repaired, or trigger apoptosis, programmed cell death, when damage cannot be mended. In healthy tissue the rate of cell division is balanced by the rate of cell death, so tissue size stays constant and cells are replaced only as fast as they are lost.

What earns the marks

• Stating that the cell cycle is regulated at checkpoints, such as before DNA replication, before mitosis, or before chromatids separate.
• Giving a specific checkpoint function: checking cell size, checking DNA has replicated completely, checking DNA is undamaged, or checking spindle attachment.
• Explaining that some genes code for proteins that stimulate division while others code for proteins that halt division or trigger apoptosis.
• Stating that the rate of cell division is balanced against the rate of cell death so that tissue size is maintained.
• Explaining that damaged cells are destroyed by apoptosis rather than being allowed to divide.

Common mistakes and how to put them right

• Writing that mitosis is controlled by the nucleus without naming a checkpoint, a gene or apoptosis.

  Put it right: Name a specific checkpoint, gene or process.

• Confusing control of the cell cycle with a controlled variable in an experiment because the question uses the word control.

  Put it right: In this context control means regulation of the cycle.

• Swapping the roles of genes that stimulate division and genes that halt it.

  Put it right: State clearly which type stimulates and which type halts the cycle.

• Describing apoptosis as accidental cell damage and death rather than programmed cell death.

  Put it right: Apoptosis is a controlled, programmed process.

• Saying cells divide when the organism needs them to, with no mechanism given.

  Put it right: Give the mechanism, such as checkpoint control or gene regulation.

Exam tips

• Give a checkpoint a job: it is not enough to say the cycle is checked, you must say what is checked.
• Link control back to genes whenever a question mentions mutation; the mutation must affect a gene controlling the cycle.