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4.3.1Additional lessons

Gene Mutations — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 106 of 142

📚 IB Biology SL 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 5Types of Gene Mutation

Genes are sequences of bases, so any change to the order of those bases can change the information they carry.

Gene mutation
  • Gene mutation A change in the sequence of bases in a gene, which may create a new allele of that gene.
Substitution
  • Substitution One base is replaced by a different base, so the length of the gene stays the same.
Insertion
  • Insertion One or more extra bases are added into the sequence, making the gene longer.
Deletion
  • Deletion One or more bases are lost from the sequence, making the gene shorter.
Point mutation
  • Point mutation A change affecting a single base, which includes single-base substitutions, insertions and deletions.
MutationOriginalMutated
SubstitutionATG CCAATG CGA
InsertionATG CCAATG TCCA
DeletionATG CCAATG CA

Part 2 of 5Causes and Randomness

Causes and Randomness
Copying errors
  • Copying errors Mistakes occasionally occur when DNA is replicated during the S phase of interphase, and a wrong base may be inserted.
Radiation
  • Radiation High-energy radiation, such as ultraviolet light and X-rays, can damage DNA and alter bases.
Chemical mutagens
  • Chemical mutagens Some chemicals, including components of tobacco smoke, react with DNA and change its base sequence.
Random
  • Random Mutations occur by chance and at any time; we cannot predict which gene will change.

Part 3 of 5From Mutation to Allele

1
A base change occurs in a gene, either through a copying error or damage from radiation or chemicals.
2
If the change is not repaired before the DNA is replicated again, the new sequence is copied.
3
The mutated sequence may specify a different amino acid sequence in the protein the gene codes for.
4
The different version of the gene is a new allele, which can sometimes be inherited.
5
Natural selection then acts on the allele, favouring it, selecting against it or having no effect.

Part 4 of 5Worked Example

Question

  • An mRNA reads AUG AAA GCU CCU, coding for Met-Lys-Ala-Pro.
  • A single G is inserted immediately after AUG.
  • Write the new codons and the amino acids they specify.

  1. Original sequenceAUGAAAGCUCCU.
  2. Insert G after the first three bases: AUG G AAAGCUCCU, so the sequence is AUGGAAAGCUCCU.
  3. Regroup into codonsAUG GAA AGC UCC U.
  4. AUG is Met, GAA is Glu, AGC is Ser and UCC is Ser.

Answer The new sequence is Met-Glu-Ser-Ser, so every amino acid after the insertion has changed (a frameshift).

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.
  • Substitution The replacement of one base with another.
  • Insertion The addition of one or more bases to a sequence.
  • Deletion The loss of one or more bases from a sequence.
  • Mutagen A factor, such as radiation or a chemical, that increases the rate of mutation.

Common Mistakes

  • Saying mutations are only caused by radiation; copying errors during replication are also common.
  • Believing organisms can choose or direct which mutations arise; they are random.
  • Mixing up the terms insertion and substitution.
  • Using mutation and allele as if they mean the same; a mutation can create a new allele.

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