18.2 - Mutations

What a mutation is

• A mutation is a random change to the DNA - a change to the sequence of bases, or to the structure of a gene.
• Mutations happen continuously and at random - for example, when DNA isn't copied perfectly as a cell prepares to divide.
• A mutation can sometimes be passed on from a parent to its offspring.
• A mutation produces a genetic variant - a different form of the gene, also called a new allele.

How mutations affect the phenotype

• The phenotype is the organism's observable characteristics.
• Most mutations have no effect on the phenotype at all - these are described as neutral.
• Some have a small effect, altering a characteristic only slightly.
• Very rarely, a single mutation has a significant effect and produces a noticeably different characteristic.
• The ones that change anything obvious are a tiny minority of all the mutations an organism carries.

What causes mutations

• Mutations happen spontaneously - often when DNA isn't copied perfectly as a cell divides.
• They are random: you can't predict where in the DNA a mutation will occur.
• The mutation rate (how many mutations occur in a given time) is not fixed - certain things in the environment increase it.

Factors that raise the mutation rate

• Ionising radiation - Such as X-rays, gamma rays, and ultraviolet rays. The larger the dose, the greater the chance of a mutation.
• Mutagens - Certain chemicals, such as the chemicals found in tobacco.

Why exposure is dangerous

• These factors don't guarantee a mutation - they only make one more likely.
• Raising the mutation rate also raises the chance of a harmful mutation, which is why exposure to them is dangerous.
• Reducing exposure lowers the chance of mutations occurring.