Gene expression and mutations (HT ONLY)

Not all parts of DNA code for proteins. Non-coding parts of DNA play a role in gene expression and can switch genes on and off. Variations in non-coding regions may affect how genes are expressed.
A mutation is a change in the DNA sequence; for example, an insertion of a nucleotide or a deletion of one.
Mutations occur continuously, but most mutations do not alter the protein or only alter it slightly so that its shape and therefore function is not changed.
A few mutations code for an altered protein with a different shape, altering the activity of a protein, e.g. an enzyme may no longer fit the substrate-binding site or a structural protein may lose its strength.
Some mutations are advantageous and allow genetic variation.

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