CourseCourseQ BankQuestion BankDocsDocuments

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
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–3

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 2025Paper2 ["HL"] · TZ15(c)[ 2 ]D1.3.4—Causes of mutation
November 2018Paper2 ["HL"] · TZ01(g)[ 3 ]D1.3.4—Causes of mutation
November 2018Paper2 ["HL"] · TZ01(b)[ 2 ]D1.3.4—Causes of mutation
May 2017Paper2 ["HL"] · TZ25(a)(i)[ 1 ]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 marks
How 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

[Maximum number: 2]

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.