AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Variation: Mutations occur continuously.
A mutation is a random change in the base sequence of DNA, and because DNA is copied whenever cells divide, new mutations arise continuously in every population.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 4 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A mutation is a random change in the base sequence of DNA, and because DNA is copied whenever cells divide, new mutations arise continuously in every population. Most occur during DNA replication before mitosis or meiosis, or are caused by mutagens such as ultraviolet radiation or tar in cigarette smoke. A concrete example is a substitution in a gene controlling a protein, changing one triplet of bases so a different amino acid is inserted. Because mutation is random, it can happen in any cell at any time, and the rate can be increased by exposure to mutagens. Most mutations have no effect on the phenotype, some are harmful, and a few are beneficial; the key idea is that new alleles appear continually, providing the genetic variation on which natural selection can act.
What earns the marks
- A mutation is a change in the base sequence of DNA, so it creates a new allele.
- Mutations happen at random and arise continuously, including during DNA replication before cell division.
- Mutagens such as ultraviolet radiation or chemicals in cigarette smoke increase the mutation rate.
- Most mutations have no effect on the phenotype; some are harmful and a few are beneficial.
- Mutation is the original source of new genetic variation in a population.
Common mistakes and how to put them right
Thinking mutations are always harmful.
Put it right by stating that most have no phenotypic effect and only some are harmful.
Believing organisms mutate because they need to adapt.
Put it right by stressing that mutation is random and not directed by need.
Confusing mutation with natural selection.
Put it right by separating the random origin of a new allele from the non-random survival of organisms that carry it.
Exam tips
- Define mutation precisely as a change in the base sequence of DNA, not simply as a change in a gene.
- Use the word random when explaining when and where mutations occur.
- Link mutation to variation by stating that it produces new alleles for natural selection to act on.
Try the lesson
The start of the Variation: Mutations occur continuously. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement best describes how often mutations occur in the DNA of living organisms?
Listen
A mutation is a random change in the base sequence of DNA, and because DNA is copied whenever cells divide, new mutations arise continuously in every population. Most occur during DNA replication before mitosis or meiosis, or are caused by mutagens such as ultraviolet radiation or tar in cigarette smoke. A concrete example is a substitution in a gene controlling a protein, changing one triplet of bases so a different amino acid is inserted. Because mutation is random, it can happen in any cell at any time, and the rate can be increased by exposure to mutagens. Most mutations have no effect on the phenotype, some are harmful, and a few are beneficial; the key idea is that new alleles appear continually, providing the genetic variation on which natural selection can act. A strong answer earns marks for these points. A mutation is a change in the base sequence of DNA, so it creates a new allele. Mutations happen at random and arise continuously, including during DNA replication before cell division. Mutagens such as ultraviolet radiation or chemicals in cigarette smoke increase the mutation rate. Most mutations have no effect on the phenotype; some are harmful and a few are beneficial. A common slip: Thinking mutations are always harmful. Instead, stating that most have no phenotypic effect and only some are harmful. Exam tip: Define mutation precisely as a change in the base sequence of DNA, not simply as a change in a gene.
Key words
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Mutation
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Mutagens
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Randomness of mutation
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Sources of genetic variation
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See every definition
- Mutation
- A random change in the base sequence of DNA, which creates a new allele.
- Mutagens
- Agents such as ultraviolet radiation or tar in cigarette smoke that increase the rate at which mutations occur.
- Randomness of mutation
- Mutation can happen in any cell at any time and is not directed by need.
- Sources of genetic variation
- New alleles appear continually through mutation, providing the genetic variation on which natural selection can act.
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