Uncontrolled cell division can lead to the formation of tumours and of…

The idea

A mutation changes the base sequence of DNA. This may change the sequence of amino acids in the polypeptide coded for, which can alter the tertiary structure of the protein and so its function. When the gene affected controls the cell cycle, the protein that should regulate division may no longer work correctly, checkpoints are ignored and the cell divides rapidly and uncontrollably. The mass of cells produced is a tumour. A benign tumour grows slowly, stays inside a capsule in one place and is often harmless. A malignant tumour grows rapidly, is not encapsulated, invades and destroys surrounding tissue, and sheds cells into the blood or lymph that seed secondary tumours elsewhere, a process called metastasis. That is cancer.

What earns the marks

• A change in the DNA base sequence or triplet caused by mutation.
• A resulting change in the sequence of amino acids, and so in the primary and tertiary structure of the protein.
• Rapid or uncontrollable cell division, or for cell division that can no longer be regulated.
• The mass of cells forming a tumour, with malignant tumours invading surrounding tissue or spreading to form secondary tumours.
• The mutation affecting a gene that controls the cell cycle.

Common mistakes and how to put them right

• Writing that the protein is not formed, or that different amino acids are formed.

  Put it right: The credited idea is a change in the sequence of amino acids.

• Writing growth or cell reproduction where the question requires cell division or cell replication.

  Put it right: Use the term cell division.

• Treating every tumour as a cancer, when only malignant tumours are cancerous.

  Put it right: Benign tumours are not cancerous.

• Saying the cell wants to divide or decides to divide instead of saying regulation has been lost.

  Put it right: State that regulation has been lost.

• Describing metastasis as the tumour growing larger rather than as cells spreading to a new site.

  Put it right: Metastasis is the spread of cells to form secondary tumours.

Exam tips

• Build the chain in order, DNA base sequence to amino acid sequence to tertiary structure to loss of function to uncontrolled division; each link can carry a mark.
• Use the word division rather than growth when describing the process, and when comparing benign and malignant tumours give the same feature for both sides, for example rate of growth, capsule or invasion.