Gene mutations might arise during DNA replication. The deletion and substitution of bases. Gene mutations occur spontaneously. The mutation rate is increased by mutagenic agents. Some mutations result in a different amino acid sequence in the encoded polypeptide. Due to the degenerate nature of the genetic code, not all mutations result in a change to the amino acid sequence of the encoded polypeptide. The rate of cell division is controlled by proto-oncogenes that stimulate cell division and tumour suppressor genes that slow cell division. A mutated proto-oncogene, called an oncogene, stimulates cells to divide too quickly. A mutated tumour suppressor gene is inactivated, allowing the rate of cell division to increase.

When a cell of the body is creating a new cell (mitosis) it's DNA is copied, a mistake could happen resulting in a new cell with a mutation in it's DNA.

A base might be left out. This means that every codon read after the missing base would be wrong because the reading of the code has been shifted along and triplets will be read with different bases (known as a frame shift). This affect more codons the earlier it is in the code.

A base might be substituted with another base:

• A mis-sense mutation is if the changed base makes a codon code for a different amino acid
• A nonsense mutation is if it makes a stop codon then during translation amino acids will only be added to the chain up to that point making a shorter protein
• A silent mutation is if it makes a new codon that codes for the same amino acid (so the proteins made are not affected)

The primary structure of a protein (the amino acids) determines the tertiary/quaternary structure of a protein, so a change in the primary structure could result in a protein that is the wrong shape to carry out its function.

Mutations are spontaneous, this means they happen randomly. However, the rate that they happen at can be increased by certain things, like some radiation and some chemicals, these are called mutagenic agents.

Mutations can occur on the section of DNA that codes for proteins which regulate cell division. These genes are proto-oncogenes which make proteins that stimulate cell division and tumour suppressor genes which make proteins that slow cell division.

Mutations on these genes result in oncogenes which create proteins that permanently activate protein receptors on the cell surface membrane that stimulate the cell to divide too fast and inactive tumour suppressor cells. This means cells will divide too often and so not just replace dead cells, but create extra cells leading to a tumour.