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2.4.1Unit 2: Cells, Development, Biodiversity and Conservation

Mutations — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 21 of 26 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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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.
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Part 1 of 5Nature of Gene Mutations

All point mutations involve a change at a single position in the DNA base sequence.

Gene mutation
  • Gene mutation A change in the sequence of bases in a gene, which can alter the polypeptide that the gene codes for.
Spontaneous origin
  • Spontaneous origin Mutations occur by chance, mostly as errors during DNA replication,
    • and some are caused by mutagens such as ultraviolet or ionising radiation.
Substitution
  • Substitution One base is swapped for a different base, and only the triplet containing that base is changed.
Insertion
  • Insertion One or more extra nucleotides are added into the sequence, which can alter every codon downstream.
Deletion
  • Deletion One or more nucleotides are lost from the sequence, which can also alter every codon downstream.
Chromosome mutations
  • Chromosome mutations Whole genes may be duplicated or lost, and chromosomes can fail to separate properly in meiosis.

Part 2 of 5Types of Substitution

Types of Substitution
Silent
  • Silent The new triplet codes for the same amino acid because the code is degenerate, so the polypeptide is unchanged.
Missense
  • Missense The new triplet codes for a different amino acid, so one amino acid in the polypeptide is changed.
Nonsense
  • Nonsense The new triplet is a stop codon, so translation ends early and a shortened polypeptide is made.
Sickle cell example
  • Sickle cell example A single substitution in the β-globin gene changes glutamic acid to valine, and so alters the shape of haemoglobin.

Part 3 of 5Frameshift Mutations

1
The genetic code is read in non-overlapping groups of three bases, starting from the start codon.
2
Insertion or deletion of one base moves the reading frame, so every codon after the mutation is read differently.
3
The amino acid sequence after the change is usually completely different, and a premature stop codon may appear.
4
Insertion or deletion of three bases in a row adds or removes one amino acid but leaves the rest of the frame intact.
5
The effect on the phenotype depends on where in the gene the change happens and on how the protein's shape is altered.
MutationEffect on later codonsLikely effect on protein
Silent substitutionNoneNone
Missense substitutionNoneOne amino acid changed
Nonsense substitutionNoneShortened polypeptide
Insertion or deletion of 1 baseAll shiftedOften non-functional

Part 4 of 5Worked Example

Question

  • An mRNA reads AUG GGU CCA UAA.
  • The first G of the second codon is deleted.
  • Write the new codons and the first three amino acids, given that GUC = Val and CAU = His.

  1. Original codonsAUG | GGU | CCA | UAA.
  2. Deleting the first G of GGU leaves AUG GUC CAU AA, regrouped into threes.
  3. The new codons are AUG, GUC, CAU, followed by the remaining bases.
  4. The amino acids are Met, Val and His; the original stop codon UAA is no longer read in frame.

Answer

  • New sequence AUG GUC CAU;
    • the polypeptide begins Met–Val–His and is altered after the deletion.

Part 5 of 5Exam Checklist & Tips

2.4.1 – Key Points for the Exam

  • Mutation A change in the base sequence of DNA.
  • Substitution Replacement of one nucleotide by a different nucleotide.
  • Frameshift A shift in the reading frame caused by an insertion or deletion not in multiples of three.
  • Mutagen An agent that increases the rate of mutation, such as ultraviolet radiation.
  • Silent mutation A base change that does not alter the amino acid sequence.

Common Mistakes

  • Saying that all mutations are harmful, when many are silent or neutral and a few are beneficial.
  • Stating that a substitution shifts the reading frame, when only insertions and deletions can do so.
  • Writing that mutations are caused only by mutagens, when they also arise spontaneously.
  • Confusing a nonsense mutation, which makes a stop codon, with a missense mutation, which changes an amino acid.

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