Mutations

• What is a mutation?

  A change in the base sequence of DNA 
• Mutations can occur spontaneously in our cells, but certain factors can increase the risk.
• Which of the following can increase the risk of mutations?

  • Ionising radiation such as x-rays and gamma rays
  • Carcinogens like those found in cigarette smoke 
• Will exposure to ionising radiation always cause a mutation?

  no

  Ionising radiation is only a risk factor, so just increases the chance of a mutation occurring. 
• A triplet, or a codon, is a set of three bases which codes for one amino acid.
• A mutation to one of the bases in DNA can change the amino acid that a triplet (codon) codes for. 
• What is the most likely result of a genetic mutation?

  No significant change to the organism 

  Most mutations don't have any significant effect, only very rarely will they impact the organism. 
• Most mutations occur in

  Non-coding DNA
• Non-coding DNA often controls the expression of coding DNA.

  What does it mean for a gene to be expressed?

  It's turned on, so the proteins it codes for will be produced 
• What happens in a substitution mutation?

  One base in the sequence is replaced with a different base 
• Enzymes are proteins that speed up the rate of chemical reactions.

  Describe how a change of one of the bases in the gene for an enzyme could affect the functioning of that enzyme.

  
  

  • Change in a base changes the triplet changes the amino acid it codes for So there will be a different sequence of amino acids. This means the polypeptide will fold into a differently shaped enzyme
  • The enzyme may have a differently shaped active site So it may no longer be complementary to the substrate Which would mean it couldn't form an enzyme-substrate complex So it couldn't catalyse the reaction as well
• How many triplets (codons), and thus amino acids are affected by a substitution mutation?

  1
• What happens in an insertion mutation?

  An extra base is added into the sequence 
• Which type of mutation usually has a larger impact on the protein: substitution mutations, or deletion mutations?

  Deletion mutations 

  
  

  Deletion and insertion mutations have a much larger effect than substitution mutations as they disrupt all of the triplets after the mutation.