What mistakes do students make with Alteration of the sequence of bases in DNA can alter the?

Writing that an antibiotic, pesticide or pathogen caused the mutation. The mutation is spontaneous and pre-existing. Stating that organisms mutate to adapt or because they need to, which implies purposeful change rather than random mutation. Using the term 'gene' where 'allele' is required to describe the new genetic variant.

AQA · A-Level · Biology

Course version: 7402 · Version from Sept 2016

Gene mutations occur spontaneously.

Full specification wording

Alteration of the sequence of bases in DNA can alter the: Gene mutations occur spontaneously.

Spontaneous mutations happen by chance during normal cell activity, primarily as errors in DNA replication, without external causes or relation to an organism's needs.

Checked against the AQA 7402 specification (opens in a new tab) on 19 September 2026.

Point 2 of 8

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

State that mutations occur at random or by chance, primarily as errors during DNA replication.
Establish that the mutation occurs before, and independently of, any environmental selection pressure.
Identify that the mutation produces a new allele, creating genetic variation within the population.
Explain that selection pressures act on existing variation, increasing the survival and reproductive success of individuals with the advantageous allele.
Conclude that the frequency of the advantageous allele increases over many generations.

Common mistakes and how to put them right

Writing that an antibiotic, pesticide or pathogen caused the mutation.
Put it right: The mutation is spontaneous and pre-existing.
Stating that organisms mutate to adapt or because they need to, which implies purposeful change rather than random mutation.
Using the term 'gene' where 'allele' is required to describe the new genetic variant.
Saying the 'number' of alleles increases rather than the 'frequency' or 'proportion' of the allele.
Assuming mutation only happens when a mutagen is present, ignoring the natural spontaneous mutation rate.

Exam tips

Write natural selection answers in a fixed chronological order: mutation, new allele, survival advantage, reproduce and pass on the allele, allele frequency increases over generations.
Never let the selection pressure appear before the mutation in your sentence order.
Explicitly state 'over many generations' when describing a change in allele frequency.

Try the lesson

The start of the Alteration of the sequence of bases in DNA can alter the: Gene mutations occur spontaneously. lesson, made from checked teaching notes for this specification point.

Have a go first

Gene mutations are described as spontaneous. What does this mean?

Tip: press 1 to 4 to choose

Listen

Spontaneous mutations happen by chance during normal cell activity, primarily as errors in DNA replication, without external causes or relation to an organism's needs. Every gene has a low, measurable natural mutation rate. This is crucial for evolution, as variation must exist in a population before a selection pressure arises. For example, an allele conferring antibiotic resistance appears by random mutation. The antibiotic acts as a selection pressure, determining which individuals survive and reproduce. Consequently, the frequency of the advantageous allele increases over generations. The pressure does not create the allele; organisms do not mutate to adapt. A strong answer earns marks for these points. State that mutations occur at random or by chance, primarily as errors during DNA replication. Establish that the mutation occurs before, and independently of, any environmental selection pressure. Identify that the mutation produces a new allele, creating genetic variation within the population. Explain that selection pressures act on existing variation, increasing the survival and reproductive success of individuals with the advantageous allele. Exam tip: Write natural selection answers in a fixed chronological order: mutation, new allele, survival advantage, reproduce and pass on the allele, allele frequency increases over generations.

See it

A vertical flowchart with seven boxes connected by arrows. The first box reads 'Spontaneous mutation occurs by chance' and points to 'New allele created', which points to 'Genetic variation in the population'. That points to 'Selection pressure applied', which points to 'Individuals with advantageous allele survive and reproduce'. This points to 'Allele passed on', which finally points to 'Frequency of advantageous allele increases over many generations'. — It shows the chronological order from a spontaneous mutation creating a new allele, through selection acting on existing variation, to the frequency of the advantageous allele increasing over many generations.

Put it in order

Put these steps in the order they happen.

1. Frequency of advantageous allele increases over many generations
2. Allele passed on
3. Genetic variation in the population
4. Individuals with advantageous allele survive and reproduce
5. Spontaneous mutation occurs by chance
6. New allele created
7. Selection pressure applied

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A vertical flowchart with seven boxes connected by arrows. The first box reads 'Spontaneous mutation occurs by chance' and points to 'New allele created', which points to 'Genetic variation in the population'. That points to 'Selection pressure applied', which points to 'Individuals with advantageous allele survive and reproduce'. This points to 'Allele passed on', which finally points to 'Frequency of advantageous allele increases over many generations'.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

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