Gene Mutations and Their Effects — AQA A-level Biology

SPEC 3.8.1 - GENE EXPRESSION — MUTATIONS

Gene Mutations

Types of mutation (substitution, deletion, insertion, inversion, duplication, translocation), mutagenic agents, and effects on the encoded polypeptide

What is a Gene Mutation?

A mutation is a change to the genetic material. Gene mutations involve a change to the base sequence of DNA.

Gene Mutation

A change in the base sequence of DNA.

When Do Mutations Arise?

Gene mutations can arise spontaneously during DNA replication — errors in base pairing that are not corrected by proofreading mechanisms.

They occur at a low but constant background rate in all organisms.

The mutation rate can be increased by mutagenic agents (e.g. ionising radiation, UV light, certain chemicals).

Mutagenic Agent

A factor that increases the rate of mutation, e.g. ionising radiation, UV light, some chemicals.

⚠️ EXAMINER ALERT: Mutations are random and spontaneous — they are not directed by the environment. A mutagenic agent increases the rate of mutation but does not ensure mutations are beneficial or targeted at a particular gene.

Types of Gene Mutation

AQA specifies six types of gene mutation. You must know all six, but the most exam-tested are substitution and deletion/insertion (frameshift).

TypeWhat happensEffect on codons downstream?

SubstitutionOne base replaced by a different baseOnly ONE codon changed

DeletionOne base removed from the sequenceFrameshift — ALL codons downstream altered

Insertion (Addition)One extra base inserted into the sequenceFrameshift — ALL codons downstream altered

InversionA sequence of bases is reversedVariable — depends on sequence length

DuplicationA sequence of bases is repeatedVariable — extra copies of bases present

TranslocationA sequence of bases moves to a different location in the genomeVariable — depends on new location

Key Exam Point

Deletions and insertions are almost always more damaging than substitutions because they cause a frameshift — every codon from the mutation site onwards is changed.

Base Substitution: Three Possible Outcomes

A base substitution replaces one base with another. It alters only the codon at the mutation site. There are three possible consequences:

Silent (Synonymous) Mutation

The changed codon still codes for the same amino acid. No change to the polypeptide sequence. Possible because the genetic code is degenerate — most amino acids are coded for by more than one codon.

Missense Mutation

The changed codon codes for a different amino acid. The primary structure of the polypeptide is altered at one position. This may change the tertiary structure and therefore the function of the protein.

Nonsense Mutation

The changed codon becomes a stop codon (UAA, UAG, or UGA). Translation terminates prematurely. A truncated (shorter) polypeptide is produced — usually non-functional.

Substitution: Visualised

The diagram below shows how a single base substitution (C → A at position 4) affects the codons read during translation.

Note: The sequence shown is the coding strand (5'→3'). A single base substitution changes only the codon in which it occurs. All other codons remain identical.

Deletion & Insertion: The Frameshift

When a base is deleted or inserted, the entire reading frame shifts. Every codon downstream of the mutation is altered.

Frameshift Mutation

A mutation (insertion or deletion) that changes the reading frame of all codons downstream of the mutation site, resulting in a completely different — usually non-functional — polypeptide sequence from that point onward.

Why Frameshifts Are So Damaging

A substitution affects one amino acid at most (and may be silent).

A deletion or insertion shifts the reading frame: every triplet from that point onward encodes a different amino acid.

The resulting polypeptide has a completely different sequence from the mutation site to the end.

Often a premature stop codon is also created → truncated protein.

The protein is almost always non-functional.

Exam Technique

When asked to compare substitution and deletion, always state: "A deletion causes a frameshift, altering ALL codons downstream, whereas a substitution only changes ONE codon." This is the key distinction AQA tests repeatedly.

Frameshift: Visualised

Deletion of a single base (base 4, C) shifts the reading frame for all subsequent codons.

Inversion, Duplication & Translocation

These three mutation types involve larger changes to sequences of bases. AQA requires you to know their definitions.

Inversion

A sequence of bases is reversed within the DNA. For example, the sequence ATCGGT might become TGGCTA. The affected codons will encode different amino acids.

Duplication

A sequence of bases is repeated. Extra copies of one or more codons are present. This can produce a protein with additional amino acid sequences inserted.

Translocation

A sequence of bases moves to a different location in the genome — sometimes to a different chromosome. This can disrupt the original gene and may activate a gene in the new location inappropriately.

Exam Tip

For inversion, duplication, and translocation, AQA does not require you to trace every effect on amino acid sequence in detail. Focus on knowing the definition of each. The detailed mechanism questions are reserved for substitution and deletion/insertion.

The Degenerate Genetic Code

Understanding why a substitution mutation can be silent (no change to the polypeptide) requires understanding degeneracy.

Degenerate Genetic Code

Most amino acids are encoded by more than one codon. There are 64 possible codons but only 20 amino acids (plus 3 stop codons), so many codons are synonymous — they encode the same amino acid.

Why Degeneracy Protects Against Mutation

The third base of many codons is the most variable — a change here often does not change the amino acid encoded.

Example: GGU, GGC, GGA, GGG all encode glycine. A substitution at the third position (e.g. GGU → GGC) is silent.

A substitution mutation can therefore be harmless if the new codon encodes the same amino acid.

This is NOT true for deletions/insertions — the frameshift affects all downstream codons regardless of degeneracy.

Exam Language

Always say the genetic code is "degenerate" — not "redundant" (which implies unnecessary). AQA mark schemes use "degenerate code" — this exact phrase is expected.

Mutagenic Agents

Mutagenic agents increase the rate of mutation. They do not direct which mutations occur — they increase the probability of errors during DNA replication or directly damage DNA bases.

Mutagenic AgentTypeMechanism

Ionising radiation (X-rays, gamma rays)PhysicalBreaks DNA strands; causes base modifications that lead to mispairing during replication

UV lightPhysicalCauses adjacent thymine bases to form thymine dimers — covalent bonds between adjacent thymines → distorts helix → mispairing

Chemical mutagens (e.g. alkylating agents, base analogues)ChemicalBase analogues are incorporated in place of normal bases; alkylating agents modify bases so they pair incorrectly during replication

⚠️ EXAMINER ALERT: Students often write that UV light "breaks DNA." UV light specifically causes thymine dimers — it does NOT break the sugar-phosphate backbone. Ionising radiation (X-rays, gamma rays) breaks DNA strands. Keep these distinct.

DEEP Framework: Effects of a Deletion Mutation

Exam question: "Explain how a deletion mutation could result in a non-functional protein." [4 marks]

DDefine: A deletion mutation is the removal of a base from the DNA sequence. This causes a frameshift — a change in the reading frame of all codons downstream of the mutation site.

EExplain mechanism: During transcription, the altered DNA template produces an mRNA with a changed codon sequence from the mutation site onwards. During translation, tRNA molecules with different anticodons bind, incorporating a different sequence of amino acids into the polypeptide from that point.

EEvidence/consequence: The primary structure of the protein is altered from the mutation site to the C-terminus. Because the sequence of amino acids determines the folding pattern (secondary and tertiary structure), the tertiary structure is likely to be different. Active site shape (if an enzyme) or binding site (if a receptor or carrier protein) is no longer complementary to the substrate/ligand.

PPractical/Quantitative: A frameshift also frequently generates a premature stop codon downstream of the mutation site. The polypeptide is truncated — it lacks the complete functional domain. The combined effect (wrong sequence + premature termination) means the protein almost always cannot carry out its biological role.

From Mutation to Altered Protein Function

The critical chain to understand: mutation in DNA → change in mRNA codon → change in amino acid sequence → change in protein structure → change in protein function.

The Complete Pathway

Mutation alters DNA base sequence at the gene locus

Altered template strand → different mRNA codon(s) produced during transcription

Different tRNA anticodon base-pairs at ribosome → different amino acid incorporated

Primary structure altered → different sequence of R-groups

Hydrogen bonds, ionic bonds, disulfide bridges form at different positions → altered secondary and tertiary structure

Shape of active site / binding site / structural domain is different → protein may be non-functional

Not All Mutations Affect Function

Silent substitution: same amino acid → same protein → no change in function

Missense substitution in non-critical region: different amino acid but not at active site / binding site → function preserved

Frameshift or nonsense mutation: almost always non-functional due to widespread change or truncation

Mutations & the Cell Cycle: Link to Cancer

Some of the most medically significant mutations occur in genes that control cell division.

Synoptic Stretch — Spec 3.8.4 (Lesson 8.04)

Proto-oncogenes: genes that normally promote cell division in a controlled way. A mutation can convert them into oncogenes — uncontrolled division results.

Tumour suppressor genes: genes that normally limit cell division. A mutation inactivating a tumour suppressor gene removes this brake — division continues unchecked.

Cancer usually requires multiple mutations in the same cell — both a proto-oncogene mutation AND a tumour suppressor mutation typically needed.

This content is covered in full in Lesson 8.04. At this stage, know that mutations in cell-cycle control genes are a key mechanism underlying cancer.

Synoptic Link

This content links forward to Lesson 8.04 (Gene Expression & Cancer). In Paper 3 synoptic essays you may be asked to discuss the relationship between mutations, gene expression, and cancer. Ensure you can name the gene types: proto-oncogene, oncogene, tumour suppressor gene (e.g. p53).

Synoptic Link — Topic 2

Mutations in genes controlling the cell cycle connect to Topic 2.04 (The Cell Cycle & Mitosis). Cancer cells divide by uncontrolled mitosis — the normal checkpoint controls of the cell cycle fail. Mutagenic agents (ionising radiation, UV light) are also used in cancer treatment to kill rapidly dividing cells.

Examiner Alerts: Common Errors

⚠️ ERROR 1 — Mutation language: Never write "the mutation produces a different protein." The correct chain is: mutation → altered mRNA codon → different amino acid in primary structure → different tertiary structure → different function. The examiner wants mechanism, not just outcome.

⚠️ ERROR 2 — "All mutations are harmful": This is incorrect. Silent mutations cause no change in amino acid sequence. Even missense mutations may not affect protein function if the amino acid change is conservative (similar R-group) or not in a critical region.

⚠️ ERROR 3 — Confusing DNA triplets, mRNA codons, tRNA anticodons: A DNA triplet on the template strand is complementary to the mRNA codon. The tRNA anticodon is complementary to the mRNA codon (not directly to the DNA). Work through any base-change question in stages: DNA → mRNA codon → tRNA anticodon → amino acid.

⚠️ ERROR 4 — "Degenerate" vs "redundant": AQA specifically uses the term "degenerate" to describe the genetic code (multiple codons per amino acid). Never use "redundant" — it will not score the mark.

Practice Question

A section of DNA has the template strand base sequence:

3'— TAC — CCG — ATG — GTA — 5'

A deletion mutation removes the base at position 4 (the first base of the second codon).

(a) State what is meant by a frameshift mutation. [1 mark]

(b) Explain how this deletion mutation could result in a non-functional protein. [4 marks]

(c) A scientist proposes that a substitution mutation at position 6 (G → A) might have no effect on the encoded polypeptide. Explain why this is possible. [2 marks]

a)

A frameshift mutation is a deletion or insertion that shifts the reading frame, so that all codons downstream of the mutation encode different amino acids. (1 mark)

D:

The deletion of a base causes a frameshift — all codons from the mutation site to the 3' end are altered.

E:

During transcription, the altered template strand produces mRNA with a different codon sequence. During translation, different tRNA anticodons base-pair with the altered codons, incorporating different amino acids into the polypeptide from position 2 onwards. (1 mark)

E:

The primary structure of the polypeptide is changed downstream of the mutation. This leads to a different pattern of hydrogen bonds and ionic bonds forming, altering the tertiary structure of the protein. The active site (or binding site) is no longer the correct complementary shape. (2 marks)

c)

The genetic code is degenerate — most amino acids are encoded by more than one codon. If the substitution at position 6 produces a new codon that encodes the same amino acid, the primary structure is unchanged, so no change in protein function results. (2 marks)

Summary: Gene Mutations

Key Points

ConceptKey Fact

Gene mutationA change in the base sequence of DNA

Spontaneous mutationsArise during DNA replication; low background rate

Mutagenic agentsIncrease mutation rate (ionising radiation, UV light, chemicals)

SubstitutionOne base replaced; ONE codon altered; may be silent, missense, or nonsense

Deletion / insertionFrameshift — ALL codons downstream altered; usually non-functional protein

Silent mutationPossible because genetic code is degenerate

Nonsense mutationCreates stop codon → truncated polypeptide

Must-Know Exam Language

"Frameshift" — not just "all bases shift"

"Degenerate" genetic code — not "redundant"

Name the levels: primary structure → tertiary structure → active site shape

Comparison: Sub vs Deletion

Substitution: one codon changed, may be silent

Deletion: frameshift, all downstream codons altered, almost always non-functional

Exam Tip

In any question about mutation effects, work through the full mechanism: mutation in DNA base sequence → altered mRNA codon → different amino acid in primary structure → altered tertiary structure → changed active/binding site → non-functional protein. Never skip steps — each named level can earn a mark.