A mutation is a change in the sequence or number of nucleotides in DNA, which is passed on from a parent to their offspring.

A nucleotide is one of the smaller molecules that make up a nucleic acid (DNA or RNA).

It is made up of a nucleotide base (nitrogenous base), a five-carbon sugar and at least one phosphate group.

Mutations play an important role in the process of evolution.

A mutation may affect only one nucleotide base.

This type of mutation is known a point mutation or a single base substitution.

For example, one of the nucleotide bases may be substituted for another, or a nucleotide base may be added or deleted.

Other mutations affect large parts of a chromosome or even an entire chromosome. These are known as chromosome abnormalities or chromosome aberrations.

Some types of chromosome abnormalities are

• Deletions of part of a chromosome
• Duplications of part of a chromosome
• Inversions, in which part of a chromosome is turned around, so that its genes are in reverse order.
• Insertions of extra nucleotides into a chromosome
• Translocations of one part of a chromosome to the end of another chromosome.

Other chromosome abnormalities involve having the wrong number of chromosomes. Either a chromosome is missing or there is an extra chromosome.

Down's syndrome, also known as trisomy 21, is a disorder of human beings caused by the presence of an extra 21st chromosome or part of an extra 21st chromosome.

Many mutations fall between these two extremes.

Mutations can have various effects.

Visible mutations affect the outward appearance of the individual. The mutation that causes albinism is an example of a visible mutation.

Nutritional mutations affect the individual's ability to produce a molecule that is necessary for growth, such as a particular amino acid.

A lethal mutation, which causes the death of the individual, commonly prevents an individual from producing a protein that is necessary for life.

Microorganisms can develop mutations that make them resistant to a particular toxin. For example, some bacteria have evolved to become resistant to certain antibiotics.