AHL: Mutation and gene editing
IB Biology HLΒ· Theme D: D.3 (AHL)Β· 45 min read
1. Types of Mutationβ β ββββ± 15 min
Mutation
A permanent, heritable change in the nucleotide sequence of DNA, that can occur spontaneously or be induced by external agents.
Example:
A single base change in the HBB gene causes sickle cell anemia.
Mutations are categorized by their scale and the cell type they occur in:
Gene (point) mutations: Affect one or a small number of nucleotides within a single gene
Chromosome mutations: Affect large segments of chromosomes, altering chromosome structure
Somatic mutations: Occur in body cells, not passed to offspring
Germline mutations: Occur in gametes, inherited by future generations
Explain why a 2-nucleotide insertion in a coding sequence usually has a larger phenotypic effect than a single base substitution.
- 1
The genetic code is read in non-overlapping triplet codons, each coding for one amino acid.
- 2
A single base substitution only changes one codon, so only one amino acid in the polypeptide is altered. It may even be a silent mutation if the new codon codes for the same amino acid.
- 3
A 2-nucleotide insertion shifts the entire reading frame of all codons downstream of the insertion.
- 4
This changes every amino acid after the insertion, almost always producing a non-functional protein.
- 5
Thus, the frameshift insertion almost always causes a much larger change to phenotype.
Exam tip:
Always specify the exact type of mutation (e.g. base substitution) instead of only the general 'point mutation' for full marks.
2. Causes and Consequences of Mutationsβ β β βββ± 20 min
Mutations arise from two main sources: spontaneous errors during DNA replication, and induced damage from external mutagens. Common mutagens include ultraviolet radiation, ionizing radiation, and chemical carcinogens found in tobacco smoke.
Mutagen
An external agent that damages DNA and increases the rate of mutation.
Explain how a base substitution in the HBB gene causes sickle cell anemia.
- 1
The HBB gene codes for the beta-globin subunit of hemoglobin, the protein that carries oxygen in red blood cells.
- 2
A single base substitution changes the codon GAG (coding for hydrophilic glutamic acid) to GTG (coding for hydrophobic valine).
- 3
When oxygen levels are low, the altered hemoglobin sticks together to form long fibers that distort red blood cells into a rigid sickle shape.
- 4
Sickle-shaped cells block capillaries, causing pain and organ damage, and are broken down rapidly leading to anemia.
- 5
The mutation is recessive, so only homozygous individuals develop the disease; heterozygotes have sickle cell trait and are resistant to malaria.
3. CRISPR-Cas9 Gene Editing Mechanismβ β β β ββ± 25 min
CRISPR-Cas9
A programmable gene editing technology that can cut DNA at a specific target sequence, allowing genes to be disrupted (knocked out) or inserted.
CRISPR-Cas9 is derived from the natural immune system of bacteria, which store fragments of viral DNA to recognize and cut invading viral DNA in future infections. Researchers adapted this system to edit any target DNA sequence.
Outline how CRISPR-Cas9 is used to knock out a defective disease-causing gene.
- 1
A guide RNA is designed to have a complementary sequence to the target defective gene.
- 2
The guide RNA and Cas9 nuclease enzyme are delivered into the patient's affected cells.
- 3
The guide RNA binds to the target defective gene, directing Cas9 to cut both strands of DNA at the target site.
- 4
The cell repairs the broken DNA via non-homologous end joining, which introduces small random insertions or deletions that disrupt the gene sequence, knocking out its function.
- 5
Edited cells are transplanted back into the patient, reducing symptoms caused by the defective gene.
Test your understanding:
Which component of the CRISPR-Cas9 system is responsible for cutting target DNA?
Guide RNA
Cas9 enzyme
DNA polymerase
Ligase
Reveal answer
1 βCorrect! Cas9 is the nuclease enzyme that cuts DNA. The guide RNA only targets the correct sequence.
4. Applications and Ethical Considerationsβ β β β ββ± 20 min
Gene editing has wide applications in medicine, agriculture, and basic research. In medicine, it is used to develop treatments for genetic disorders, blood cancers, and viral infections like HIV. In agriculture, it creates crops with improved yield, disease resistance, and nutritional content.
5. Common Pitfalls
Wrong move:
Calling all small mutations just 'point mutation' without specifying the type.
Why:
IB exam markers require specific identification of mutation type for full marks.
Correct move:
Name the exact type: base substitution, insertion, deletion, frameshift, etc.
Wrong move:
Claiming all mutations are harmful.
Why:
Most mutations are neutral, and rare beneficial mutations generate genetic variation for evolution.
Correct move:
State that mutations can have neutral, harmful, or beneficial effects depending on their impact and environment.
Wrong move:
Confusing somatic and germline gene editing in ethical questions.
Why:
Heritability is the core difference that changes ethical arguments, so mixing this up loses marks.
Correct move:
Always specify that somatic editing is non-heritable, while germline editing alters the genome of future generations.
Wrong move:
Describing CRISPR as only cutting DNA, ignoring the repair step.
Why:
The functional outcome of editing depends on how the cell repairs the cut, not just cutting itself.
Correct move:
Explain that non-homologous end joining knocks out genes, while homology-directed repair inserts new sequences.
6. Quick Reference Cheatsheet
Category | Type | Key Details |
|---|---|---|
Gene Mutation | Base substitution | One base replaced; usually small effect |
Gene Mutation | Frameshift insertion/deletion | <3 bases added/removed; almost always large effect |
Chromosome Mutation | Deletion | Loss of large chromosome segment |
Chromosome Mutation | Translocation | Segment moves to non-homologous chromosome |
Gene Editing | CRISPR-Cas9 | Guide RNA targets sequence, Cas9 cuts DNA |
Gene Editing | Somatic editing | Non-heritable; treats disease in patients |
Gene Editing | Germline editing | Heritable; ethically controversial in humans |
7. Frequently Asked
What is the difference between a gene and chromosome mutation?
A gene mutation affects one or a small number of nucleotides within a single gene, while a chromosome mutation affects large segments of chromosomes, changing the structure or number of whole chromosomes.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Mutation types and CRISPR ethics
- 2023 Β· 1
Base substitution effects in HBB gene
- 2024 Β· 2
Outline CRISPR-Cas9 mechanism
Going deeper
What's Next
Understanding mutation and gene editing builds on your core knowledge of genetics and connects to key themes in evolutionary biology and modern biotechnology. Mutations are the ultimate source of genetic variation, so this topic is foundational for studying natural selection and speciation. Gene editing is a rapidly advancing field that regularly appears in IB exam extended response questions, requiring you to connect biological concepts to real-world ethical debates. Building on this topic, you will next explore broader applications of biotechnology in agriculture and medicine, as well as the role of mutation in evolution.