Gene Mutations
A gene mutation is a change in the nucleotide sequence of DNA. Mutations are the ultimate source of all genetic variation: without them, evolution could not occur, populations could not adapt to changing environments, and the diversity of life would have remained vanishingly small. At the same time, mutations are the molecular cause of inherited disease and the engine of carcinogenesis. Understanding the types of mutation, their molecular consequences, and the cellular machinery that prevents or repairs them is therefore central to both evolutionary biology and medicine.
Spec mapping: This lesson sits in AQA 7402 Section 3.4.3 — Genetic diversity can arise as a result of mutation or during meiosis. The relevant content covers the types of point mutations (substitution, insertion, deletion), the effects of mutations on protein structure and function, the role of mutagens, and the relationship between mutation and natural selection. (Refer to the official AQA specification document for exact wording.)
What Is a Gene Mutation?
Key Definition: A gene mutation (or point mutation) is a change in one or a small number of nucleotide bases in a DNA sequence. This may alter the sequence of amino acids in the polypeptide encoded by that gene, with downstream consequences for protein structure, cellular function and organismal phenotype.
Key points:
- Mutations can occur spontaneously during DNA replication (due to errors by DNA polymerase that escape proofreading) or can be induced by external mutagens (chemical, physical, biological).
- The rate of spontaneous mutation is very low — approximately 1 in 10⁹ bases per replication in humans — because of the 3′→5′ proofreading activity of DNA polymerase and the post-replication mismatch repair machinery.
- Most mutations are neutral (have no detectable effect on the organism). Some are deleterious (causing disease or impairment), and a small fraction are beneficial (providing a selective advantage). The classification depends on environmental context: a mutation that is deleterious in one habitat may be neutral or beneficial in another.
- Somatic mutations arise in body cells and affect only the individual; they are not inherited by offspring. Skin cancers driven by UV-induced mutation are typical somatic events.
- Germline mutations arise in gametes or in the cells that produce them; they can be passed to offspring and become heritable variation in the population.
- Timing matters. A mutation that occurs in an early embryonic cell affects a large proportion of the adult body (mosaicism); a mutation in a single cell of the adult gut lining affects only that cell's descendants.
Types of Point Mutation
1. Substitution Mutations
A substitution is where one base pair is replaced by another. There are three possible consequences:
a) Silent (Synonymous) Mutation
- The base change produces a different codon that still codes for the same amino acid.
- This is possible because the genetic code is degenerate — most amino acids are coded for by more than one codon.
- Silent mutations most often occur at the third position (wobble position) of a codon.
- No effect on the polypeptide or protein function.
Example: If the mRNA codon changes from GCU to GCC, both code for alanine — no change in the protein.
b) Missense Mutation
- The base change produces a codon that codes for a different amino acid.
- The effect on protein function depends on:
- The chemical properties of the original and new amino acids.
- The position of the change in the protein (e.g., in the active site of an enzyme vs. on the surface).
- A conservative missense mutation substitutes an amino acid with a chemically similar one (minimal effect).
- A non-conservative missense mutation substitutes an amino acid with a chemically different one (potentially severe effect).
c) Nonsense Mutation
- The base change creates a premature stop codon (UAA, UAG, or UGA).
- This causes translation to terminate early, producing a truncated (shortened) polypeptide.
- Truncated proteins are usually non-functional and are often rapidly degraded.
- Nonsense mutations typically have severe effects on phenotype.
2. Insertion Mutations
- An extra base pair is inserted into the DNA sequence.
- This shifts the reading frame of all codons downstream of the insertion — a frameshift mutation.
- Every codon after the insertion is read differently, so the amino acid sequence downstream is completely altered.
- Frameshift mutations usually result in a non-functional protein because:
- Many amino acids are changed.
- A premature stop codon is often encountered, truncating the protein.
3. Deletion Mutations
- A base pair is removed from the DNA sequence.
- Like insertion, this causes a frameshift mutation affecting all downstream codons.
- The consequences are the same as for insertion mutations: the reading frame is disrupted, producing a radically different and usually non-functional protein.
Comparison of Mutation Types
| Mutation Type | Mechanism | Reading Frame | Effect on Polypeptide |
|---|
| Silent substitution | One base replaced | Unchanged | No change — same amino acid |
| Missense substitution | One base replaced | Unchanged | One amino acid changed — effect varies |
| Nonsense substitution | One base replaced | Unchanged | Premature stop codon — truncated protein |
| Insertion | Extra base added | Shifted (frameshift) | All downstream amino acids altered |
| Deletion | Base removed | Shifted (frameshift) | All downstream amino acids altered |
Exam Tip: Frameshift mutations (insertion and deletion) are generally more damaging than substitution mutations because they affect every codon downstream of the mutation, whereas a substitution affects only one codon.
Sickle Cell Disease — A Case Study
Sickle cell disease is caused by a single missense mutation in the gene for the beta-globin chain of haemoglobin (on chromosome 11).
The Mutation
- In the normal allele (HbA), the sixth codon of the beta-globin mRNA is GAG, coding for glutamic acid.
- In the sickle cell allele (HbS), the sixth codon is mutated to GUG, coding for valine.
- This is a single base substitution (A → U in the mRNA, corresponding to T → A in the DNA template strand).
Consequences
- Molecular level: Valine is a hydrophobic amino acid, whereas glutamic acid is hydrophilic and negatively charged. This changes the surface properties of the haemoglobin molecule.
- Protein level: Under low oxygen conditions, the hydrophobic valine residues on adjacent HbS molecules interact, causing the haemoglobin molecules to polymerise into long, rigid fibres.
- Cellular level: The haemoglobin fibres distort the red blood cell into a characteristic sickle (crescent) shape.
- Organism level: Sickled red blood cells:
- Are less flexible and can block capillaries, restricting blood flow (causing pain crises and tissue damage).
- Have a shorter lifespan (leading to anaemia).
- Are less efficient at carrying oxygen.
Heterozygote Advantage
- Individuals who are heterozygous (HbA HbS) — carriers with sickle cell trait — are generally healthy but have a degree of protection against malaria.
- The malaria parasite (Plasmodium falciparum) is less able to complete its life cycle in sickle-shaped red blood cells.
- This is why the HbS allele is maintained at high frequency in populations where malaria is endemic — an example of balanced polymorphism and a heterozygote advantage (also called overdominance).
Exam Tip: Sickle cell disease is a perfect example of how a single base substitution can have effects at every level of biological organisation — from the molecular to the whole-organism level. Be prepared to explain each level in a structured answer.
Mutagens
Key Definition: A mutagen is any agent that increases the rate of mutation above the natural (spontaneous) rate.
Types of mutagen:
Chemical Mutagens
- Base analogues (e.g., 5-bromouracil) — structurally similar to normal bases and are incorporated during DNA replication, but pair incorrectly, causing substitution mutations.
- Deaminating agents (e.g., nitrous acid) — remove amino groups from bases, changing their pairing properties (e.g., deamination of cytosine produces uracil, which pairs with adenine instead of guanine).
- Intercalating agents (e.g., ethidium bromide, acridine dyes) — insert themselves between base pairs, distorting the helix and causing insertion or deletion mutations during replication.
- Alkylating agents (e.g., mustard gas, some chemotherapy drugs) — add alkyl groups to bases, causing mispairing or blocking replication.
Physical Mutagens
- Ionising radiation (e.g., X-rays, gamma rays) — high-energy radiation that can break covalent bonds in DNA, causing strand breaks, deletions, and rearrangements.
- UV radiation — causes adjacent thymine bases to form thymine dimers (covalent bonds between adjacent thymines on the same strand), which distort the DNA helix and block replication. This is repaired by nucleotide excision repair, but if unrepaired, can lead to mutations and skin cancer.
Biological Mutagens
- Some viruses can insert their DNA into the host genome, disrupting genes.
- Transposable elements ("jumping genes") can move within the genome and disrupt gene function.
DNA Repair Mechanisms
Cells have evolved several mechanisms to repair DNA damage and reduce mutation rates. The relative contributions of these systems explain why the net mutation rate is so low despite the enormous number of replication events and chemical assaults that DNA undergoes daily:
- Proofreading by DNA polymerase — the enzyme tests each newly added nucleotide and, if incorrectly paired, removes it via its 3′→5′ exonuclease activity before continuing synthesis. This reduces the error rate from ~10⁻⁵ to ~10⁻⁷.
- Mismatch repair (MMR) — post-replication, MMR proteins (MSH2, MLH1 in humans) detect mismatched base pairs that escaped proofreading, excise the newly synthesised strand around the mismatch, and resynthesise it correctly. This further reduces the error rate to ~10⁻⁹. Inherited defects in MMR cause Lynch syndrome (hereditary non-polyposis colorectal cancer).
- Base excision repair (BER) — repairs damaged bases (e.g. deaminated cytosines, oxidised guanines). A DNA glycosylase removes the damaged base; an AP endonuclease cleaves the backbone; DNA polymerase and ligase fill the gap.
- Nucleotide excision repair (NER) — removes bulky lesions such as thymine dimers caused by UV light. A section of the damaged strand (~25 nucleotides) is excised and replaced using the complementary strand as template. Defects in NER cause xeroderma pigmentosum, a condition with extreme UV sensitivity and a >1000-fold increased risk of skin cancer.
- Double-strand break repair — repairs the most dangerous form of damage. Homologous recombination uses the sister chromatid as template (accurate, restricted to S/G2 phase); non-homologous end joining ligates broken ends directly (faster but error-prone). Defects in BRCA1, BRCA2 or ATM compromise homologous recombination.
Defects in DNA repair mechanisms increase mutation rates and cancer risk. Mutations in BRCA1 and BRCA2 impair homologous recombination repair and are associated with substantially increased lifetime risk of breast, ovarian and other cancers. Mutations in MMR genes (MSH2, MLH1, MSH6, PMS2) cause Lynch syndrome with elevated risk of colorectal, endometrial and other cancers.
Summary
- Gene mutations include substitutions (silent, missense, nonsense) and insertions/deletions (causing frameshifts).
- Sickle cell disease results from a single missense substitution in the beta-globin gene.
- Mutagens (chemical, physical, biological) increase mutation rates.
- DNA repair mechanisms reduce the impact of mutations.
- Mutations are the ultimate source of genetic variation.
Exam Tip: In extended response questions about mutations, always distinguish between the type of mutation, its effect on the mRNA codons, the amino acid sequence, protein structure and function, and ultimately the phenotype. This multi-level (DNA → RNA → protein → cell → organism) explanation is expected for full marks.
Mutation Rates and the Mutation–Selection Balance
The spontaneous mutation rate in humans is approximately 1 × 10⁻⁹ mutations per base pair per cell division — astonishingly low given the ~6 × 10⁹ bases in a diploid genome. Even so, every newly conceived human being carries an estimated 50–100 new (de novo) mutations not present in either parent. Most of these fall in non-coding DNA and have no detectable effect; a small fraction occur in exons; an even smaller fraction alter protein function.
At the population level, the equilibrium frequency of a deleterious allele is set by the balance between: