What is a base substitution?

A mutation in which one base is replaced by a different base.

What is a base deletion?

A mutation in which one base is lost from the DNA sequence.

Why does a deletion often have a greater effect than a substitution?

It shifts the way bases are grouped into triplets, so every later codon may change.

Why do not all base substitutions change the amino acid sequence?

The genetic code is degenerate, so the new codon may code for the same amino acid.

What happens if a substitution forms a stop codon?

Translation ends early and the polypeptide is shorter.

What is a mutagenic agent?

A factor that increases the rate of gene mutation.

Give two examples of mutagenic agents.

Ultraviolet radiation, ionising radiation (X-rays) or certain chemicals.

What is chromosome non-disjunction?

Failure of chromosomes to separate in meiosis, so gametes have extra or missing chromosomes.

Gene mutationsAQA A-Level Biology: Flashcards

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What is a gene mutation?

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What is a gene mutation?: A change in the base sequence of DNA.
When can gene mutations arise spontaneously?: During DNA replication.
What is a base substitution?: A mutation in which one base is replaced by a different base.
What is a base deletion?: A mutation in which one base is lost from the DNA sequence.
Why does a deletion often have a greater effect than a substitution?: It shifts the way bases are grouped into triplets, so every later codon may change.
Why do not all base substitutions change the amino acid sequence?: The genetic code is degenerate, so the new codon may code for the same amino acid.
What happens if a substitution forms a stop codon?: Translation ends early and the polypeptide is shorter.
What is a mutagenic agent?: A factor that increases the rate of gene mutation.
Give two examples of mutagenic agents.: Ultraviolet radiation, ionising radiation (X-rays) or certain chemicals.
What is chromosome non-disjunction?: Failure of chromosomes to separate in meiosis, so gametes have extra or missing chromosomes.
When does non-disjunction arise?: Spontaneously, during meiosis.
How many chromosomes would a human zygote have if a gamete with 24 fuses with a normal gamete?: 47 (24 + 23).

Exam questions on Gene mutations

Part of the mRNA transcribed from a gene reads AUG GAA CCU AAA UGG. The relevant codons are: AUG methionine (start); GAA and GAG glutamic acid; GUA valine; CCU and CCC proline; AAA lysine; UGG tryptophan.
The mRNA codon GAA is changed to GAG by a base substitution. A different substitution changes GAA to GUA. Explain why the first substitution has no effect on the polypeptide, whereas the second does.2 marks
A microbiologist grew two cultures of the same bacterium for many generations. Culture X was exposed to ultraviolet radiation throughout. Culture Y was kept in the dark. Both cultures were then checked for changes in the base sequence of one gene. A small number of mutant cells were found in culture Y and many more in culture X.
Compare the rate of mutation in the two cultures and explain the difference.2 marks
The mRNA from a normal allele reads AUG CAA GGA UUC GAA UGA. The mRNA from a mutant allele reads AUG CAA UGA UUC GAA UGA. The relevant codons are: AUG methionine (start); CAA glutamine; GGA glycine; UUC and UUU phenylalanine; GAA glutamic acid; UGA stop.
Identify the type of gene mutation shown and explain its effect on the polypeptide.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).