Mutation

In the syllabus and the exam

Mutation is a random, spontaneous or induced change in an organism's DNA or chromosome structure, and the syllabus distinguishes gene mutation (a change within a single gene's DNA sequence, such as a base being added, deleted or substituted) from chromosomal mutation (a larger-scale change affecting the structure or number of whole chromosomes). Candidates are expected to name mutagens, factors that increase the rate of mutation, such as ultraviolet light, ionising radiation (X-rays, gamma rays) and certain chemicals, and to distinguish mutations occurring in body (somatic) cells, which affect only the individual and are not passed on, from mutations occurring in reproductive (germline) cells, which can be inherited by offspring. A frequent error is assuming all mutations are harmful; while many mutations are neutral (no noticeable effect on the phenotype) or harmful (disrupting normal protein function, as in sickle cell anaemia), a small proportion are beneficial and provide the new variation on which natural selection can act, which links this content standard directly to the earlier chapter on variation. Candidates should also be able to explain, at a basic level, why a mutation in the DNA sequence coding for a protein can change that protein's shape, and therefore its function, connecting the topic back to gene structure and protein synthesis studied in earlier chapters rather than treating mutation as an isolated, unexplained event. Candidates should further note that a mutation altering a single base in a gene's DNA sequence does not always change the resulting protein, since some changes have no effect on the amino acid the affected codon specifies.

Example

Exposure to high-energy radiation can cause a random change in a gene's DNA sequence, a gene mutation, which may alter the structure of the protein that gene codes for and, in turn, change the organism's phenotype.

Related terms

VariationGeneChromosome

Source:SRC-DSKP-EN