IB Biology · Theme D: Continuity and change · SL and HL
D1.3 Mutation and gene editing
A one-page summary of D1.3 Mutation and gene editing, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How do gene mutations occur?
What are the consequences of gene mutation?
What D1.3 covers
Types of gene mutation
- D1.3.1Three ways to change a gene
- D1.3.2One base swapped — everything, or nothing
- D1.3.3When the reading frame shifts
Causes and consequences of mutation
- D1.3.4Where mutations come from
- D1.3.5Random, but not equal
- D1.3.6Which cell it happens in matters
- D1.3.7The only source of new alleles
HL — gene editing technology
- D1.3.8 · HLBreaking a gene on purpose
- D1.3.9 · HLA guide RNA finds the target
- D1.3.9 · HLFrom a cut to a cure
- D1.3.10 · HLSome sequences barely change at all
D1.3 Mutation and gene editing: summary
Types of mutation
- A gene mutation is a permanent change to a gene's nucleotide sequence: substitution, insertion, or deletion.
- Insertions/deletions change the gene's length; a substitution does not.
Substitutions & SNPs
- A SNP is a one-base difference produced by a substitution.
- Degeneracy of the code means a substitution can be silent (GAG→GAA), missense (GAG→GUG, sickle-cell disease) or nonsense (GAG→UAG, a stop codon).
Indels & frameshift
- An indel not a multiple of 3 causes a frameshift, misreading every downstream codon.
- Major or in-frame indels can still disable a protein by disrupting folding.
Causes & randomness
- Mutagens: chemicals (5-bromouracil, nitrosamines) and radiation (UV, ionizing) — plus uncorrected replication/repair errors.
- Mutations occur randomly across the genome — never deliberately targeted to a trait.
Germline, somatic & variation
- Germline mutations are inherited; somatic mutations are not, but can cause cancer.
- Mutation is the original source of all genetic variation — essential, long-term, for evolution.
HL · Gene editing
- Gene knockout disables a gene to study its function; CRISPR-Cas9 uses a guide RNA to direct Cas9 to cut one target site (e.g. Casgevy, for sickle-cell disease).
- Conserved sequences persist via functional constraint and/or a slower mutation rate.
Key terms
- Frameshift mutation
- A change in the reading frame of a gene, caused by an insertion or deletion whose length is not a multiple of three.
- SNP
- Single-nucleotide polymorphism — a single-base difference from the usual sequence at a given position, produced by a substitution mutation.
- Mutagen
- A chemical or physical agent that damages DNA or increases the rate of mutation.
- Germline mutation
- A mutation in a gamete-forming cell that can be inherited by offspring.
- Somatic mutation
- A mutation in a body cell that is not passed to offspring, but can affect that cell's mitotic descendants.
- Gene knockout HL
- A technique that deliberately disables a gene to investigate its normal function.
- Guide RNA HL
- A short RNA sequence, complementary to a target DNA site, that directs the Cas9 enzyme to cut there.
- Conserved sequence HL
- A DNA or protein sequence that stays identical or similar across species, or over evolutionary time.
Sample exam questions
Three of the 32 multiple-choice questions for D1.3. Try each one before opening the answer.
Question 1. Which change is a substitution mutation?
- One or more nucleotides are removed from the sequence
- One nucleotide is replaced by a different nucleotide
- One or more nucleotides are added to the sequence
- A codon is duplicated in the same reading frame
Show the answer
Answer: B. A substitution replaces a single nucleotide with another; insertions add nucleotides and deletions remove them.
Question 2. A base substitution changes a codon from AAA to AAG. Both codons specify lysine. Which feature of the genetic code explains this outcome?
- Non-overlapping structure
- Universality
- Degeneracy
- Continuity
Show the answer
Answer: C. Degeneracy means that more than one codon can specify the same amino acid.
Question 3. Why does a one-nucleotide deletion typically have a greater effect than a three-nucleotide deletion?
- It shifts the reading frame
- It changes only one codon
- It cannot be repaired
- It deletes an entire gene
Show the answer
Answer: A. One nucleotide is not a multiple of three, causing a frameshift; three nucleotides remove exactly one codon.
Practise D1.3
Study notes, every question and full markschemes for D1.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.