4.3.2Additional lessons
Consequences of Gene Mutations — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 107 of 142
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 4Effects of Substitutions
The consequence of a substitution depends on the codon that is created and the role of the amino acid affected.
Silent
- Silent The new codon still codes for the same amino acid, so the protein is unchanged;
- for example GAA to GAG both code for glutamic acid.
Missense
- Missense The new codon codes for a different amino acid, as when GAG becomes GUG and glutamic acid is replaced by valine.
Nonsense
- Nonsense The new codon is a stop codon, for example UGG for tryptophan becoming UGA, which ends the polypeptide early.
SNP
- SNP A single-nucleotide polymorphism is a base that varies between individuals and is usually harmless but can affect disease risk.
| Substitution | Codon change | Result |
|---|---|---|
| Silent | GAA → GAG | Glu → Glu |
| Missense | GAG → GUG | Glu → Val |
| Nonsense | UGG → UGA | Trp → stop |
Part 2 of 4Insertions and Deletions
Because the genetic code is read in triplets, the number of bases added or lost matters as much as their identity.
1
Bases are read in groups of three, so adding or removing one or two bases shifts the reading frame.
2
Every codon after the change is read incorrectly, giving a different sequence of amino acids, which is a frameshift.
3
A frameshift often creates an early stop codon, producing a short, non-functional polypeptide.
4
Adding or removing three bases, or a multiple of three, keeps the reading frame and only adds or removes whole amino acids.
5
Frameshifts are therefore usually more damaging than single base substitutions.
Part 3 of 4Where the Mutation Happens
Whether a mutation matters to future generations depends on the type of cell in which it occurs.
Where the Mutation Happens
Somatic cells
- Somatic cells Mutations in body cells affect only that cell and its daughter cells, and are not passed to offspring.
Germ cells
- Germ cells Mutations in cells that make gametes may be passed to offspring, who then carry the change in all their cells.
Cancer link
- Cancer link Mutations in somatic cells that control cell division can lead to tumours.
Beneficial cases
- Beneficial cases Rarely a mutation produces a protein that gives an advantage, and the allele can spread through a population.
Part 4 of 4Exam Checklist & Tips
4.3.2 – Key Points for the Exam
- Silent mutation A mutation that does not change the amino acid sequence.
- Missense mutation A mutation that changes one amino acid.
- Nonsense mutation A mutation that creates a stop codon.
- Frameshift A change in the reading frame caused by an insertion or deletion not in multiples of three.
- Germ cell A cell that gives rise to gametes.
- Somatic cell A body cell that does not form gametes.
Common Mistakes
- Thinking every substitution changes the protein; silent mutations do not.
- Forgetting that deleting three bases does not shift the reading frame.
- Saying somatic mutations are inherited by children.
- Calling all mutations harmful.
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