Gene Mutations

A gene mutation is a change in the nucleotide sequence of DNA. Mutations are the ultimate source of all genetic variation and therefore the raw material for natural selection and evolution. This lesson covers the types of gene mutations, their effects on protein structure, the example of sickle cell disease, and the role of mutagens — all key topics for AQA A-Level Biology.

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What Is a Gene Mutation?

Key Definition: A gene mutation (or point mutation) is a change in one or a small number of nucleotide bases in a DNA sequence. This may alter the sequence of amino acids in the polypeptide encoded by that gene.

Key points:

• Mutations can occur spontaneously during DNA replication (due to errors by DNA polymerase that escape proofreading) or can be induced by mutagens.
• The rate of spontaneous mutation is very low (~1 in 10⁹ bases per replication in humans) because of the proofreading ability of DNA polymerase and DNA repair mechanisms.
• Most mutations are neutral (have no effect on the organism). Some are harmful (cause disease), and very rarely, a mutation is beneficial (provides a selective advantage).
• Mutations in somatic cells affect only the individual and are not inherited. Mutations in gametes (or cells that produce gametes) can be passed to offspring.

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Types of Point Mutation

1. Substitution Mutations

A substitution is where one base pair is replaced by another. There are three possible consequences:

a) Silent (Synonymous) Mutation

• The base change produces a different codon that still codes for the same amino acid.
• This is possible because the genetic code is degenerate — most amino acids are coded for by more than one codon.
• Silent mutations most often occur at the third position (wobble position) of a codon.
• No effect on the polypeptide or protein function.

Example: If the mRNA codon changes from GCU to GCC, both code for alanine — no change in the protein.

b) Missense Mutation

• The base change produces a codon that codes for a different amino acid.
• The effect on protein function depends on:
  • The chemical properties of the original and new amino acids.
  • The position of the change in the protein (e.g., in the active site of an enzyme vs. on the surface).
• A conservative missense mutation substitutes an amino acid with a chemically similar one (minimal effect).
• A non-conservative missense mutation substitutes an amino acid with a chemically different one (potentially severe effect).

c) Nonsense Mutation

• The base change creates a premature stop codon (UAA, UAG, or UGA).
• This causes translation to terminate early, producing a truncated (shortened) polypeptide.
• Truncated proteins are usually non-functional and are often rapidly degraded.
• Nonsense mutations typically have severe effects on phenotype.

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2. Insertion Mutations

• An extra base pair is inserted into the DNA sequence.
• This shifts the reading frame of all codons downstream of the insertion — a frameshift mutation.
• Every codon after the insertion is read differently, so the amino acid sequence downstream is completely altered.
• Frameshift mutations usually result in a non-functional protein because:
  • Many amino acids are changed.
  • A premature stop codon is often encountered, truncating the protein.

3. Deletion Mutations

• A base pair is removed from the DNA sequence.
• Like insertion, this causes a frameshift mutation affecting all downstream codons.
• The consequences are the same as for insertion mutations: the reading frame is disrupted, producing a radically different and usually non-functional protein.

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Comparison of Mutation Types

Mutation Type	Mechanism	Reading Frame	Effect on Polypeptide
Silent substitution	One base replaced	Unchanged	No change — same amino acid
Missense substitution	One base replaced	Unchanged	One amino acid changed — effect varies
Nonsense substitution	One base replaced	Unchanged	Premature stop codon — truncated protein
Insertion	Extra base added	Shifted (frameshift)	All downstream amino acids altered
Deletion	Base removed	Shifted (frameshift)	All downstream amino acids altered

Exam Tip: Frameshift mutations (insertion and deletion) are generally more damaging than substitution mutations because they affect every codon downstream of the mutation, whereas a substitution affects only one codon.

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Sickle Cell Disease — A Case Study