explain that a gene mutation is a result of substitution or deletion or insertion of nucleotides in DNA and outline how each of these types of mutation may affect the polypeptide produced

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Protein Synthesis: Gene Mutations

Protein Synthesis

Objective

Explain that a gene mutation results from the substitution, deletion or insertion of nucleotides in DNA and outline how each type of mutation may affect the polypeptide produced.

What is a Gene Mutation?

A gene mutation is a permanent change in the nucleotide sequence of a DNA segment that codes for a protein. The change can alter the way the genetic code is read during transcription and translation, leading to variations in the amino‑acid sequence of the resulting polypeptide.

Types of Nucleotide Changes

1. Substitution (Point Mutation)

A single nucleotide is replaced by another.

  • Silent mutation: the new codon still codes for the same amino‑acid (e.g., \$GAA \rightarrow GAG\$, both code for Glutamate).

  • Missense mutation: the new codon codes for a different amino‑acid, which may alter protein function (e.g., \$GAA \rightarrow GAC\$, Glutamate → Aspartate).

  • Nonsense mutation: the new codon becomes a stop codon, truncating the polypeptide (e.g., \$UAC \rightarrow UAA\$).

2. Deletion

One or more nucleotides are removed from the DNA sequence.

  • If the number of nucleotides deleted is a multiple of three, whole codons are lost, producing a shorter polypeptide (in‑frame deletion).

  • If the deletion is not a multiple of three, the reading frame shifts (frameshift mutation), altering every downstream codon and usually creating a premature stop codon.

3. Insertion

One or more nucleotides are added into the DNA sequence.

  • Insertion of three nucleotides adds an extra amino‑acid without changing the downstream reading frame (in‑frame insertion).

  • Insertion of a number of nucleotides not divisible by three causes a frameshift, with the same consequences as a frameshift deletion.

Summary of Effects on the Polypeptide

Mutation TypeChange in DNAEffect on mRNA CodonsPossible Polypeptide Outcome
Substitution (silent)Single base replaced, codon still codes same AANo change in amino‑acid sequenceNormal polypeptide
Substitution (missense)Single base replaced, codon codes different AAOne amino‑acid alteredMay affect protein function (gain or loss)
Substitution (nonsense)Single base replaced, creates stop codonPremature termination of translationTruncated, usually non‑functional protein
Deletion (multiple of 3)One or more codons removedMissing amino‑acid(s)Shortened protein; function may be reduced
Deletion (not multiple of 3)FrameshiftAll downstream codons alteredCompletely different amino‑acid sequence; often early stop
Insertion (multiple of 3)Extra codon(s) addedAdditional amino‑acid(s)Longer protein; possible functional change
Insertion (not multiple of 3)FrameshiftAll downstream codons alteredAltered sequence with premature termination

Suggested diagram: Flow of genetic information from DNA → mRNA → polypeptide, highlighting where substitution, deletion and insertion mutations occur.

Key Points to Remember

  1. Mutations are changes in the DNA sequence; they can be spontaneous or induced.

  2. The impact of a mutation depends on its type and position within the gene.

  3. Frameshift mutations (deletions/insertions not in multiples of three) usually have the most severe effect because they alter the entire downstream reading frame.

  4. Even a single amino‑acid change (missense) can dramatically alter protein activity if it occurs at a critical site.

  5. Understanding mutation types helps predict possible phenotypic consequences and informs genetic disease studies.