Mutations | Edexcel International AS Biology Revision Notes 2018

Nature of Mutations

• Gene mutation: A change in the DNA base sequence.
  • Can alter the polypeptide sequence as the gene's DNA sequence determines amino acid order.
• Spontaneous occurrence during DNA replication.
• Mutation types:
  • Point mutations: Change in DNA at a single location.
    • Substitution
    • Insertion
    • Deletion
  • Gene/Chromosome mutations:
    • Genes replicated or lost.
    • Unequal chromosome division in meiosis, leading to extra/missing chromosomes.

Substitution of Nucleotides

• Base in the DNA sequence is randomly swapped for another.
• Alters only the amino acid in the triplet where it occurs.
• Types:
  • Silent mutations: No change in amino acid sequence.
  • Missense mutations: Alters a single amino acid (e.g., sickle cell anaemia).
  • Nonsense mutations: Creates a premature stop codon leading to incomplete proteins (e.g., cystic fibrosis).

Insertion of Nucleotides

• A nucleotide is randomly inserted into DNA, altering the original triplet.
• Causes a frameshift mutation, changing all downstream triplets.
• Can dramatically change amino acid sequence and polypeptide function.

Deletion of Nucleotides

• A nucleotide is randomly deleted.
• Also results in a frameshift mutation, affecting downstream triplets.
• Can significantly alter amino acid sequence and polypeptide function.

Effects of Mutations

• Most mutations do not significantly alter polypeptides due to the genetic code's degeneracy.
• Significantly altered polypeptides can result from a small number of mutations, affecting protein 3D structure and function.
• Rarely beneficial: Can provide evolutionary advantages (e.g., antibiotic resistance).
• Often harmful:
  • Cystic fibrosis: Non-functional chloride channels due to CFTR gene mutations.
  • Sickle-cell disease: Clumping of haemoglobin, altering red blood cell shape and function.
  • Can lead to uncontrolled cell division and cancer.
• Mutations in gametes can be inherited.
• Mutagens increase mutation likelihood:
  • Ionising radiation
  • X-rays
  • Certain chemicals

Summary

• Understanding mutations is crucial in genetics as they impact gene expression and can lead to genetic disorders or evolutionary changes. Despite being generally harmful, mutations can sometimes offer advantages, contributing to natural selection and species evolution.