Alteration of the sequences of bases in DNA

• What is a gene mutation?

  • A change in the base sequence of DNA (on chromosome)
  • Can arise spontaneously during DNA replication (interphase)
• What is a mutagenic agent?

  A factor that increases rate of mutation e.g. ultraviolet light or alpha particles
• Explain how a gene mutation can lead to the production of a non-functional protein or enzyme:

  1. Changes sequence of base triplets in DNA so changes sequence of codons on mRNA
  2. So changes the sequence of amino acids in the encoded polypeptide
  3. Changes position of bonds between amino acids
  4. So changes tertiary structure of protein
  5. Enzymes active site changes shape so substrate can't bind
  6. Enzyme-substrate complexes can't form
• Describe a substitution mutation:

  A base / nucleotide is replaced by a different base / nucleotide in DNA
• Describe an addition mutation:

  1 or more bases / nucleotides are added to the DNA base sequence
• Describe a deletion mutation:

  1 or more bases / nucleotides are lost from the DNA base sequence
• Describe a duplication mutation:

  A sequence of DNA bases / nucleotides is repeated / copied
• Describe an inversion mutation:

  A sequence of bases / nucleotides detaches from the DNA sequence, then re-joins at the same position in the reverse order
• Describe a translocation mutation:

  A sequence of DNA bases / nucleotides detaches and is inserted at a different location within the same or a different chromosome
• Explain why not all gene mutations affect the order of amino acids:

  • Some substitutions change only one triplet code / codon which could still code for the same amino acid as the genetic code is degenerate
  • Some occur in introns which do not code for amino acids as they are removed in splicing
• Explain why a change in amino acid sequence is not always harmful:

  • May not change tertiary structure of protein
  • May positively change the properties of the protein, giving the organism a selective advantage
• Explain what is meant by a frameshift:

  • Occurs when mutations changes the number of nucleotides / bases by a number not divisible by 3
  • This shifts the way the genetic code is read, so all the DNA triplets / mRNA codons downstream from the mutation change
• Explain how mutations can lead to production of shorter polypeptides:

  • Deletion or translocation -> triplets / codons missing so amino acids missing
  • Substitution, addition, deletion, duplication, inversion or translocation -> premature stop triplet / codon so amino acids at end of polypeptide