Gene mutationsAQA A-Level Biology: Mind map

Exam questions on Gene mutations

1. Part of the mRNA transcribed from a gene reads AUG GAA CCU AAA UGG. The relevant codons are: AUG methionine (start); GAA and GAG glutamic acid; GUA valine; CCU and CCC proline; AAA lysine; UGG tryptophan.
   The mRNA codon GAA is changed to GAG by a base substitution. A different substitution changes GAA to GUA. Explain why the first substitution has no effect on the polypeptide, whereas the second does.2 marks
2. A microbiologist grew two cultures of the same bacterium for many generations. Culture X was exposed to ultraviolet radiation throughout. Culture Y was kept in the dark. Both cultures were then checked for changes in the base sequence of one gene. A small number of mutant cells were found in culture Y and many more in culture X.
   Compare the rate of mutation in the two cultures and explain the difference.2 marks
3. The mRNA from a normal allele reads AUG CAA GGA UUC GAA UGA. The mRNA from a mutant allele reads AUG CAA UGA UUC GAA UGA. The relevant codons are: AUG methionine (start); CAA glutamine; GGA glycine; UUC and UUU phenylalanine; GAA glutamic acid; UGA stop.
   Identify the type of gene mutation shown and explain its effect on the polypeptide.3 marks