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Mutation is a change in a gene or chromosome. It is a rare, random change in the genetic material and it can be inherited.

Causes of mutation

Mutations occur continuously and can be spontaneous. Most are caused by errors when the DNA is copied prior to cell division.

They can also happen because of:

• ionising radiation
• chemical mutagens - such as tar from cigarette smoke

Ionising radiation includes gamma rays, X-rays and ultraviolet rays. The greater the dose of radiation a cell gets, the greater the chance of a mutation.

How mutations affect proteins

Mutations could cause a different or faulty protein to be synthesised. For example, if the protein is an important enzyme, the specific substrate might not fit into the substrate binding site. If it is a structural protein such as collagen, it might lose its strength.

However, most DNA mutations do not alter a protein. One reason is because several different triplets can code for the same amino acid. Other mutations may only alter the protein slightly so its appearance or function is not changed.

Types of mutation

There are many different types of mutations which can arise in DNA.

They include:

• point (substitution) mutations
• insertion mutations
• deletion mutations

The top row in the example below shows a length of DNA whose base sequence is CGA ACT CGA.

The diagrams show what happens to this sequence as it undergoes three separate mutations (though it is unlikely that three mutations would happen to the same sequence).

Compare each mutated version with the original length of DNA.

For each mutation below, the triplets in bold are the ones affected by the mutation.

• The top row of bases is the sequence in the original DNA.
• Triplets are CGA, ACT, CGA...
• The 2nd row shows point mutation (also known as substitution). Cytosine (C) has replaced guanine (G). This would change just one triplet - and since several triplets sometimes code for the same amino acid this might not even have an effect.
• Triplets are now CCA, ACT, CGA...
• On the 3rd row a guanine (G) has been inserted. This changes the way the bases are divided into triplets. Inserting an extra base affects every subsequent triplet as well as the triplet it is now part of. You can see this by looking at the triplets written in bold type.
• Triplets are CCG, AAC, TCG, A...
• In the 4th row a thymine (T) has been removed. This also changes the way the bases are divided into triplets. Deleting a base affects the final triplet. You can see this by looking at the triplets written in bold type.
• Triplets are CCG, AAC, CGA...

Passing mutations on

Many mutations that occur in body cells are not passed on. However, if there is a mutation in a cell which becomes a gamete, and the gamete is fertilized, then the variant caused by the mutation will be in the zygote.

The mutation may have no effect, or it could harm the embryo's development. The embryo may even die. If a mutation stops a protein working, it can cause a disease. On the other hand, the mutation may help the organism survive better.