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 type of gene mutation does not change the number of nucleotides in a gene?
- Insertion
- Substitution
- Deletion
- Translocation
Show the answer
Answer: B. A substitution replaces one nucleotide with another, so the length of the gene stays the same. Insertions and deletions change it.
Question 2. Which statement distinguishes a substitution from an insertion?
- Only an insertion changes the length of the gene.
- Only a substitution can change an amino acid.
- Only an insertion can occur during replication.
- Only a substitution is caused by mutagens.
Show the answer
Answer: A. An insertion adds one or more nucleotides, so the gene becomes longer; a substitution replaces a nucleotide and keeps the length the same.
Question 3. Casgevy is a CRISPR-Cas9 therapy for sickle-cell disease. Why is the edit not inherited by the patient’s children?
- The edit is removed when the cells divide.
- Fetal haemoglobin is not made in adults.
- Only somatic blood stem cells are edited.
- Cas9 cuts only the sickle-cell allele.
Show the answer
Answer: C. The patient’s own blood stem cells are edited outside the body and returned. Germ cells are not changed, so the edit cannot be inherited.
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.