Mutation

What mutation is

Mutation is a sudden change in the genetic material of an organism that can be passed on to offspring if it occurs in a cell that produces gametes. Mutations are the ultimate source of all new alleles, since they create genetic variation that did not exist before in a population.

Gene mutation vs chromosomal mutation

• Gene mutation, a change in the DNA base sequence within a single gene, through the substitution, insertion, or deletion of one or more bases, which can alter the protein that gene codes for.
• Chromosomal mutation, a change in the structure of a chromosome, or a change in the total number of chromosomes in a cell, affecting many genes at once rather than just one.
• A well-known example of a chromosomal mutation is Down syndrome, caused by an extra copy of chromosome 21 (trisomy 21).

Causes of mutation

Mutations can occur spontaneously, as a rare, natural error during DNA replication, or they can be induced by mutagens, physical or chemical agents that increase the rate of mutation. Common mutagens include ultraviolet light, ionising radiation such as X-rays, and certain chemicals found in tobacco smoke and some industrial substances.

Effects of mutation and how this is examined

A mutation can be harmful, if it disrupts a gene needed for normal function; beneficial, if it happens to produce a characteristic that helps an organism survive or reproduce; or neutral, if it has no noticeable effect on the phenotype at all. SPM questions commonly ask you to distinguish gene mutation from chromosomal mutation, to name mutagens and explain how they increase mutation rate, or to identify Down syndrome as an example of a chromosomal mutation involving chromosome 21.

Worked exam-style question

Question. A gardener exposed a batch of seeds to a high dose of ultraviolet light before planting them, while a second, identical batch was left untreated as a comparison. Several seedlings grown from the treated seeds showed unusual leaf shapes not seen in the untreated batch or in the parent plant.

A genetic test on one such seedling found that a single base within one gene had been substituted for a different base, and the number and structure of its chromosomes were unaffected. (a) State the type of mutation described in the treated seedling, giving a reason from the genetic test result.

(b) Name the mutagen most likely responsible for this mutation. (c) State whether this mutation would be passed on to the seedling's own offspring, and explain your answer.

(d) Suggest one way the change in leaf shape could be classified in terms of its effect on the plant.

Model answer. (a) This is a gene mutation, because only one base within a single gene was changed while the number and structure of the chromosomes remained unaffected. (b) Ultraviolet light, since the seeds were deliberately exposed to it before planting.

(c) It would be passed on only if the mutation occurred in a cell that produces gametes; a mutation that arose in the seed's own cells before or during germination could still affect the reproductive cells of the mature plant, in which case it would be inherited by the next generation. (d) The change could be beneficial, if the new leaf shape improves light capture or reduces water loss; harmful, if it reduces the plant's ability to photosynthesise; or neutral, if it has no real effect on the plant's survival or reproduction.

Practice question

Try this. A breeder keeps a large stock of hypothetical flowering plants that are all genetically identical, propagated only from cuttings. One year, a single plant in the stock unexpectedly produces flowers of a new colour, even though it received exactly the same care, soil and sunlight as every other plant.

Suggest an explanation for this observation, and state one type of mutagen that could have caused it if the plant had instead been kept beside industrial machinery emitting ionising radiation.

Exam tip

Key terms

These glossary terms connect directly to mutation:

• DNA, the molecule that stores genetic information as a sequence of bases, which a mutation alters.
• Gene, a segment of DNA coding for a characteristic; a gene mutation changes the base sequence within it.
• Chromosome, a structure of tightly coiled DNA carrying many genes; a chromosomal mutation changes its structure or number.
• Allele, one version of a gene; new alleles arise only through mutation.

Source:SRC-DSKP-EN

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