Gene mutations
BiologyΒ· 6.1(e)Β· 15 min read
1. What is a Gene Mutation?β β ββββ± 3 min
Gene Mutation
A permanent change to the nucleotide base sequence of a gene that occurs during DNA replication, altering the genetic information carried by that gene.
Example:
A mutation to the human CFTR gene causes the genetic disorder cystic fibrosis.
Gene mutations are small-scale mutations that only affect a single gene. This distinguishes them from chromosomal mutations, which alter large sections of chromosomes or whole chromosome number.
A replication error creates a mismatched base pair in DNA. Is this always classified as a gene mutation?
- 1
Recall the definition of a gene mutation requires a permanent change to the DNA base sequence.
- 2
If DNA proofreading and repair enzymes correct the mismatched base, no permanent change occurs.
- 3
If the error is not corrected, the mismatched base becomes a permanent change in the DNA sequence after the next round of replication. Only this case counts as a gene mutation.
2. Types of Gene Mutationβ β ββββ± 5 min
All gene mutations involve a change to the number or order of nucleotide bases in DNA. The three most common types are base substitution, base insertion, and base deletion.
Base Substitution
A point mutation where one nucleotide base is replaced by a different base. Only a single codon in the resulting mRNA is altered.
Original DNA template sequence: 3' TAC GTA GCA TGA 5'. A substitution changes the 6th base from T to A. What is the effect on the amino acid sequence?
- 1
Original mRNA sequence (complementary to template DNA): 5' AUG CAU CGU ACU 3'
- 2
Using the standard genetic code, the original amino acid sequence is: Met - His - Arg - Thr
- 3
The 6th base of the template is part of the second codon. New template sequence: 3' TAC GTA ACA TGA 5', so new mRNA second codon is UGU.
- 4
New amino acid sequence: Met - His - Cys - Thr. Only the second amino acid is changed, as expected for a base substitution.
Base insertion adds one or more extra nucleotides to the sequence, while base deletion removes one or more nucleotides. If the number of bases added or removed is not a multiple of 3, a frameshift occurs.
Frameshift Mutation
A shift in the triplet reading frame of all codons after the mutation site, altering every subsequent amino acid in the polypeptide.
3. Effects of Gene Mutations on Polypeptidesβ β β βββ± 5 min
Silent mutation: The mutation does not change the amino acid sequence, due to degeneracy of the genetic code.
Missense mutation: The mutation changes one amino acid to a different amino acid.
Nonsense mutation: The mutation changes an amino acid codon to a premature stop codon.
Frameshift mutation: Almost all result in a completely non-functional polypeptide.
A base substitution changes the DNA template codon CAG (codes for valine) to ATC. What type of mutation is this, and what effect will it have on the polypeptide?
- 1
mRNA codons are complementary to the DNA template strand. Original DNA CAG β mRNA GUC, which codes for valine.
- 2
Mutated DNA ATC β complementary mRNA codon is UAG, which is one of the three stop codons in the genetic code.
- 3
Since an amino acid codon was changed to a stop codon, this is a nonsense mutation.
- 4
Translation will terminate early, producing a truncated (shortened) polypeptide that is almost always non-functional.
4. Causes and Significance of Mutationsβ β β βββ± 2 min
Mutations can occur spontaneously during DNA replication, at a low natural rate. They can also be induced by external agents called mutagens.
Mutagen
A physical, chemical or biological agent that increases the rate of mutation above the spontaneous baseline.
Example:
Ionizing radiation (X-rays, UV), benzene, mustard gas, and some viruses are all common mutagens.
A student claims all mutations are harmful. Evaluate this claim.
- 1
Most random mutations do alter protein function negatively, so many are harmful.
- 2
Some mutations are silent, meaning they do not change the amino acid sequence, so they have no effect (neutral).
- 3
Rare mutations can produce new alleles that give an organism a selective advantage, for example mutation conferring antibiotic resistance in bacteria, or disease resistance in humans.
- 4
Mutations are the ultimate source of genetic variation required for evolution, so the claim that all mutations are harmful is incorrect.
5. Common Pitfalls
Wrong move:
Confusing gene mutations with chromosomal mutations
Why:
Gene mutations are small-scale changes to a single gene's base sequence, while chromosomal mutations alter whole chromosomes or large chromosome segments
Correct move:
Remember the question specifies 'gene mutation', so only describe small-scale base changes in your answer
Wrong move:
Assuming all base substitutions change the amino acid sequence
Why:
The genetic code is degenerate, so multiple codons code for the same amino acid, making some substitutions silent
Correct move:
Always check if the new codon codes for the same amino acid before describing the mutation's effect
Wrong move:
Claiming all insertions/deletions cause frameshift mutations
Why:
If the number of bases added/removed is a multiple of 3, the reading frame stays intact
Correct move:
Frameshift only occurs when the number of inserted/deleted bases is not divisible by 3
Wrong move:
Saying mutations only occur when exposed to mutagens
Why:
Mutations occur spontaneously during DNA replication at a low rate even without mutagens
Correct move:
Mutagens only increase the rate of spontaneous mutation, they do not cause all mutations
6. Quick Reference Cheatsheet
Mutation Type | Description | Effect on Polypeptide |
|---|---|---|
Base Substitution | One base replaces another | Silent, missense or nonsense; only one codon altered |
Base Insertion | One+ bases added to sequence | Frameshift if not multiple of 3; all downstream codons changed |
Base Deletion | One+ bases removed from sequence | Frameshift if not multiple of 3; all downstream codons changed |
Nonsense Mutation | Creates premature stop codon | Truncated, usually non-functional polypeptide |
Frameshift Mutation | Shifts entire reading frame | Almost always produces non-functional protein |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Describe types of gene mutation
- 2023 Β· 1
Identify effect of base substitution
- 2024 Β· 4
Explain frameshift mutation effect
Going deeper
What's Next
Gene mutations are a foundational concept for all genetic topics in CIE A-Level Biology. They are the source of new alleles, drive genetic variation and evolution, and cause many inherited genetic disorders. This topic is frequently tested in both multiple choice and structured essay questions, with common prompts asking you to compare mutation types or predict their effects. Mastering this sub-topic will prepare you for more advanced topics in inheritance, selection and genetic engineering that make up a large portion of your exam marks.