Describe how altered DNA can lead to cancer.

If a mutation occurs in the section of DNA that is the tumour suppressor gene, and if this mutation changes the amino acid sequence that the gene codes for (in most cases, there is more than one codon per amino acid - there are only 20 amino acids in total) (A codon is a sequence of three DNA or RNA nucleotides), the gene will produce a non-functioning protein. This protein will therefore not be able to do its job of stopping cells dividing or causing cells to carry out apoptosis (programmed cell death). Therefore, the tumour suppressor gene won’t be inhibiting cell division, meaning cells will divide at an uncontrollable rate by mitosis, which can lead to tumour formation.If mutation occurs in a section of DNA that is a proto oncogene, this gene may form a non-functioning version of the protein, if the mutation causes the amino acid sequence in the polypeptide to change. This now oncogene will be overactive and therefore overstimulate cell division, so the rate of division will be uncontrollable, leading to tumour formation. If the tumour formed starts invading surrounding body tissues, it becomes cancerous.