8.1 The alteration of the sequence of bases in DNA

• What is a gene mutation?


  ● A change in the base sequence of DNA

  ● Can arise spontaneously during DNA replication (interphase)
• What is a mutagenic agent?

  A factor that increases rate of mutation, eg. ultraviolet (UV) 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 sequence of amino acids in the encoded polypeptide
  3. So changes position of hydrogen / ionic / disulphide bonds (between amino acids)
  4. So changes tertiary structure (shape) of protein
  5. Enzymes - active site changes shape so substrate can’t bind, enzyme-substrate complex can’t form
• Different gene mutation

  Substitution

  • A base / nucleotide is replaced by a different base / nucleotide in DNA

  Addition

  • 1 or more bases / nucleotides added to the DNA base sequence

  Deletion

  • 1 or more bases / nucleotides lost from the DNA base sequence

  Duplication

  • A sequence of DNA bases / nucleotides is repeated / copied

  Inversion

  • A sequence of bases / nucleotides detaches from the DNA sequence, then rejoins at the same position in the reverse order

  Translocation

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

  ● Some substitutions change only 1 triplet code / codon which could still code for the same amino acid

  ○ As the genetic code is degenerate (an amino acid can be coded for by more than one triplet)

  ● Some occur in introns which do not code for amino acids
• Explain why a change in amino acid sequence is not always harmful

  ● May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don’t change)

  ● May positively change the properties of the protein, giving the organism a selective advantage
• Explain what is meant by a frameshift


  ● A frameshift occurs when gene mutations (eg. addition, deletion, duplication or translocation) change the number of nucleotides / bases by any 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

  ● The sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant

  
• What can a frameshift cause 


  • If a multiple of 3 bases is added / removed there won’t be a frameshift, but extra / less triplets will result in extra / less amino acids in the encoded polypeptide
  • A frameshift could also lead to production of a stop codon (that doesn’t code for amino acids so terminates translation), resulting in a shorter polypeptide