Alteration of DNA base sequences can alter protein structure

• Define degenerate

  Amino acids can be coded for by more than one triplet
• Define non-overlapping

  Each base is only ever part of one triplet
• Define universal

  The same triplets always code for the same amino acid in all organisms
• What are the causes of mutations

  • Mutations occur spontaneously during DNA replication when the wrong base forms a base pair and DNA polymerase joins the nucleotide to the next one
  • Mutagenic agents, such as high ionising radiation and some chemicals can increase the chances of mutations by damaging the structure of DNA, so it needs to be repaired more often, which increases the chance of a mutation occurring
• Why does the overall structure and function of a protein that a gene codes for, change when a mutation occurs?


  • The mutation changes the DNA base sequence
  • This changes the mRNA base sequence
  • This changes the amino acid sequence
  • Hydrogen, ionic bonds and disulphide bridges form in different places, so the protein has a different tertiary structure
• Describe the effect of a deletion mutation

  This causes a frameshift where all triplets, and so, all amino acids are different from the point of deletion onward. So the protein has a different amino acid sequence
• Describe the possible effects on the primary structure of a substitution mutation


  • No effect if new triplet codes for the same amino acid due to the degenerate nature of the genetic code
  • Altered if new triplet codes for a different amino acid so there is a different sequence
  • Shortened if new triplet codes for an early stop codon
• Describe an inversion mutation

  The DNA of a single gene is cut in two places and rotated 180 degrees then rejoined to the same place.

  Usually occurs during crossing over and results in a non-functional protein