D1.3.4—Causes of mutation
Mutations arise from replication errors, repair failures or mutagens such as chemicals, ultraviolet light and ionizing radiation damaging DNA in cells.
- Syllabus
- First assessment 2025
- Objective
- D1.3.4
- Level
- SL
Exam analysis
Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1B
Typical marks1
Common command terms
- State
- Explain
- Evaluate
- Identify
Scoring notes
- Common mistake
- Giving vague environmental factors without identifying radiation, chemicals or carcinogens.
Recent exam appearances
November 2025Paper1B ["SL"] · TZ14(d)(ii)[ 1 ]D1.3.4—Causes of mutation
November 2025Paper1B ["SL"] · TZ14(d)(i)[ 1 ]D1.3.4—Causes of mutation
November 2025Paper1B ["SL"] · TZ14(b)[ 1 ]D1.3.4—Causes of mutation
November 2024Paper1 ["SL"] · TZ215[ 1 ]D1.3.4—Causes of mutation
May 2019Paper2 ["SL"] · TZ24(b)[ 2 ]D1.3.4—Causes of mutation
Practice this objective
Coverage 2017–2025 · Updated 16 Jul 2026
Mutations Arise from Replication Errors and Mutagens
Gene mutations can result from errors in DNA replication or repair and from DNA damage caused by mutagens.
| Cause | Example or route to mutation |
|---|---|
| Replication error | An incorrect nucleotide escapes proofreading |
| Repair error | Damaged DNA is repaired with an altered base sequence |
| Chemical mutagen | NNK in tobacco smoke can increase DNA base-sequence changes |
| Ultraviolet radiation | UV can create abnormal links between adjacent bases |
| Ionizing radiation | X-rays or gamma rays can damage DNA, including strand breaks |
Damage is not yet a permanent mutation if accurate repair restores the original sequence. It becomes a mutation when the altered sequence remains and is copied.
A mutagen increases mutation probability; it does not produce the same mutation in every exposed cell.
Causes of mutation
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, commonly using State / Explain / Evaluate.
Command terms
State / Explain / Evaluate / Identify
What earns marks
Build the answer around this relationship: Mutagens increase the frequency of DNA sequence changes.
Watch for
Giving vague environmental factors without identifying radiation, chemicals or carcinogens.
Representative question
Question 1
Explain how chemical substances can cause cancer.
Core Mutation Effects
Gene mutations are changes in the base sequence of DNA; main types are substitution, insertion, deletion, and duplication. Base substitutions can create SNPs and change codons; degeneracy can make substitutions silent, missense, or nonsense. Insertions or deletions not in multiples of three cause frameshifts that alter downstream codons and often disrupt protein function. Mutations can arise from replication errors, repair errors, or chromosome damage; mutagens include chemicals, ionizing radiation, and ultraviolet radiation. Mutations occur randomly with respect to organism need or advantage; mutation rate varies with DNA sequence, gene expression, repair, and mutagen exposure. Germ-line mutations can be inherited by offspring; somatic mutations affect only descendant body cells and can contribute to cancer. Mutation is the original source of new alleles and genetic variation; many are neutral or harmful, but variation supplies material for natural selection.
Concept essentials
- Mutagens increase the frequency of DNA sequence changes.
- Radiation, ultraviolet light and some chemicals can cause mutations.
- Smoking can cause cancer because cigarette smoke contains mutagenic chemicals.
- Cancer can result when mutations affect genes controlling cell division.