1. In the human thyroid gland the amino acid tyrosine is converted by a series of reactions into the hormone thyroxine. Insufficient thyroxine production may lead to mental and physical retardation.

In about 1 in 5000 children, one of the enzymes involved in thyroxine production does not function effectively. The ineffective enzyme is a result of a gene mutation
(a) (i)  Give one way in which a gene mutation like this may have arisen.
  • substitution / deletion / insertion (of bases)
(ii) Give one factor which might increase the frequency at which gene mutation occurs.
  • e.g. ionising radiation / chemical mutagen
(b)  Explain how a gene mutation may result in the production of an ineffective enzyme
  • incorrect amino acid(s) would be inserted into the polypeptide chain
  • resulting in the enzyme active site being the incorrect shape for the substrate molecules




2. Mutations to DNA can affect the proteins produced by cells
(a) Explain why a base deletion mutation, may have a greater effect than a base substitution mutation
  • Substitution of a single base will only affect one amino acid.
  • This would only have a large effect if the amino acid is at a key site (e.g. active site of an enzyme)
  • Deletion of a base not only affects that codon but all codons after the deletion
  • Therefore the peptide formed may be very different from the original


(any 3)

(b) Name two agents, which can cause mutations.
  • Ionising radiation
  • Chemical mutagens
  • Viruses

(any 2)