Gene mutationsAQA A-Level Biology: Revision notes
Section 1
What is a gene mutation?
A gene mutation is a change in the base sequence of DNA. Mutations can arise spontaneously during DNA replication, when a wrong base is inserted or a base is missed. The two types you must know are base substitution and base deletion. A change in the base sequence of a gene may change the sequence of codons on the mRNA, and so the amino acid sequence of the polypeptide.
Section 2
Base substitution
In a base substitution one base is replaced by a different base. The number of bases stays the same, so only one codon is changed.
The effect depends on the codon that results:
- the new codon codes for the same amino acid: no change to the polypeptide
- the new codon codes for a different amino acid: one amino acid in the polypeptide is changed
- the new codon is a stop codon: translation ends early, so the polypeptide is shorter
Worked example: the mRNA codon GAA codes for glutamic acid. GAG also codes for glutamic acid, so GAA to GAG has no effect. GUA codes for valine, so GAA to GUA changes one amino acid.
Say which codon the substitution produces, and whether it codes for the same amino acid, a different one or a stop signal.
Section 3
Why not every substitution changes the polypeptide
The genetic code is degenerate: most amino acids are coded for by more than one triplet. Often the change is in the third base of a triplet, so the new codon still codes for the same amino acid. Because of the degenerate nature of the code, not all base substitutions cause a change in the sequence of encoded amino acids.
Do not say a substitution 'never' or 'always' changes the polypeptide. It depends on the codon formed.
Section 4
Base deletion
In a base deletion one base is lost from the sequence. All the bases after the deletion move up by one place, so the bases are read in different groups of three. This is a frameshift. Every codon after the deletion may change, so many or all of the following amino acids are altered, and a stop codon may appear early.
Worked example: normal mRNA AUG GAA CCU AAA UGG. Delete the first G of GAA and the mRNA reads AUG AAC CUA AAU GG. Every codon after AUG is different.
A deletion near the end of a gene affects fewer codons than one near the start, but it still shifts every codon after it.
Section 5
Mutagenic agents
Mutations occur spontaneously at a low rate. Mutagenic agents increase the rate of gene mutation. Examples are ultraviolet radiation, ionising radiation (such as X-rays) and certain chemicals. A mutagenic agent does not cause a mutation every time: it raises the chance that one occurs when DNA is replicated.
Section 6
Mutations in chromosome number
Mutations can also change the number of chromosomes. They arise spontaneously by chromosome non-disjunction during meiosis: a pair of homologous chromosomes (or sister chromatids) fail to separate, and both go into the same gamete.
One gamete has an extra chromosome (24 in humans) and the other is missing one (22). If a gamete with 24 is fertilised by a normal gamete with 23, the zygote has 47 chromosomes, as in Down's syndrome with three copies of chromosome 21. All cells formed from the zygote by mitosis have the same number.
Non-disjunction changes the number of chromosomes. A base substitution or deletion changes the base sequence of a gene.
That's the notes covered.
Carry on to the next subtopic.
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
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).