3.8.1 Alteration of the Sequence of Bases in DNA

• Define a gene mutation and describe two types of gene mutations that can occur. (3 marks)

  • A gene mutation is a change in the base sequence of DNA. (1)
  • Substitution: One base is replaced by another, which may or may not change the amino acid sequence due to the degenerate nature of the genetic code. (1)
  • Deletion: A base is removed from the sequence, leading to a frameshift mutation that alters all subsequent amino acids. (1)
• Describe how a deletion mutation affects the polypeptide produced. (3 marks)

  • A deletion mutation removes a base from the DNA sequence. (1)
  • This causes a frameshift, altering the reading frame of codons. (1)
  • As a result, most amino acids after the mutation are incorrect, potentially producing a non-functional protein. (1)
• Explain why not all mutations result in a change to the amino acid sequence of a protein. (4 marks)

  • The genetic code is degenerate, meaning multiple codons can code for the same amino acid. (1)
  • If a mutation is a silent mutation (e.g., a substitution that results in the same amino acid), the protein remains unchanged. (1)
  • If the mutation occurs in a non-coding region (intron), it may not affect protein production. (1)
  • Some mutations may not affect the folding or function of the protein. (1)
• A gene mutation is a change in the base sequence of DNA. These changes can occur spontaneously during DNA replication. Types of mutations can be:

  • Addition
  • Deletion
  • Substitution
  • Inversion
  • Duplication
  • Translocation
• An addition (insertion) mutation is when one or more bases are added to the sequence.
• A deletion mutation is when one or more bases are removed from the sequence.
• Addition, deletion and duplication mutations cause a frameshift. This is because they change the sequence of bases and therefore alter every triplet/codon after the mutation.
• A substitution mutation is when one base is swapped for another.

  • Can be a silent mutation
  • Only changes one triplet code and due to the degenerate nature of the genetic code, there is no change in the amino acid sequence.
• A duplication mutation is when one or more bases are repeated in the sequence.
• An inversion mutation is when a group of bases become separated from the DNA sequence and then rejoin at the same position but in the reverse order.

  • This affects the amino acid produced.
• A translocation mutation is when a group of bases become separated from the DNA sequence on one chromosome and are inserted into the DNA sequence of another chromosome.

  • This can often leas to significant effects on the phenotype.
• Causes of Mutations

  • Spontaneous mutations happen during DNA replication.
  • Mutagenic agents increase the chance of mutations, e.g.:
  • Ionising Radiation (UV, X-rays)
  • Chemicals (e.g. alcohol, benzene, tar)
• Mutations can have no effect if it occurs in a non-coding region of DNA or is a silent mutation.
• A mutation can be beneficial if it provides an advantage (e.g., antibiotic resistance in bacteria).