4.3.1Additional lessons
Gene Mutations — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 37 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Types of Gene Mutation
A gene mutation is a change in the base sequence of a gene, and it can create a new allele.
Substitution
- Substitution One base is swapped for another, so only one codon is altered and at most one amino acid in the polypeptide changes.
Insertion
- Insertion One or more extra bases are added to the sequence, which may shift the way all later codons are grouped.
Deletion
- Deletion One or more bases are lost from the sequence, which may also shift the reading frame of every codon that follows.
Frameshift
- Frameshift If the number of bases inserted or deleted is not a multiple of three, every codon downstream is read differently.
| Substitution type | Effect on amino acid | Example change |
|---|---|---|
| Silent | No change, because the code is degenerate | GAA to GAG, both glutamic acid |
| Missense | A different amino acid is inserted | GAG to GUG, glutamic acid to valine |
| Nonsense | A stop codon replaces an amino acid | CAG to UAG, chain ends early |
Part 2 of 5How Mutations Arise
1
Most mutations begin as copying errors when DNA polymerase inserts a wrong base during replication in the S phase.
2
Proofreading by DNA polymerase and mismatch repair enzymes correct most errors, but a small number escape detection.
3
Mutagens such as ultraviolet light, X-rays and some chemicals in tobacco smoke raise the rate of damage to DNA.
4
If the damaged base is copied before repair, the incorrect sequence becomes a permanent part of the daughter strand.
5
Cells that carry the change pass it on at mitosis, or to offspring if it occurs in cells that become gametes.
Part 3 of 5Features of Mutation
Features of Mutation
Random
- Random Mutations are not directed by need, so an organism cannot choose to mutate in a useful direction.
Rare
- Rare Spontaneous rates are low for any one gene, but a large genome and many cells give many new variants overall.
Somatic
- Somatic Mutations in body cells may cause cancer, but they are not inherited by children.
Germline
- Germline Mutations in gametes or their precursor cells can be inherited, and they supply the raw material for evolution.
Part 4 of 5Worked Example
Question
- The mRNA reads AUG GCA CUU.
- A deletion removes the first G of GCA.
- Write the new codons and state the type of mutation.
| Type | Description |
|---|---|
| Original sequence | A-U-G-G-C-A-C-U-U. |
| Delete the fourth base (G) | A-U-G-C-A-C-U-U. |
| Regroup into triplets | AUG CAC UU, so the last codon is incomplete. |
Answer
- A frameshift deletion;
- codons after AUG are changed (CAC replaces GCA).
Part 5 of 5Exam Checklist & Tips
4.3.1 – Key Points for the Exam
- Gene mutation A change in the base sequence of a gene.
- Missense mutation A substitution that changes one amino acid.
- Nonsense mutation A substitution that creates a premature stop codon.
- Frameshift A change of reading frame caused by insertion or deletion of bases not in multiples of three.
- Mutagen Agent that increases the rate of mutation.
Common Mistakes
- Writing that mutations are caused by the need of the organism; they occur at random.
- Assuming every substitution changes the protein, forgetting silent mutations.
- Saying that a three-base deletion always causes a frameshift; it removes one codon but keeps the frame.
- Stating that somatic mutations can be passed to offspring.
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