Gene mutationsAQA A-Level Biology: Subtopic test

1. 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.

   (a)

   The codon CCU in the mRNA is changed to CCC because one base in the DNA is replaced by another. Which term describes this type of gene mutation?

   [1 mark]

   • ABase deletion
   • BBase substitution
   • CChromosome non-disjunction
   • DBase addition

   (b)

   The first base (G) of the codon GAA is deleted from the DNA. What is the most likely effect on the polypeptide?

   [1 mark]

   • AOnly glutamic acid is replaced by a different amino acid
   • BThere is no change, because the genetic code is degenerate
   • CThe polypeptide is one amino acid longer
   • DMany of the amino acids after the deletion are changed, because the bases are read in different groups of three

   (c)

   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]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which term describes the ultraviolet radiation in this investigation?

   [1 mark]

   • AA template
   • BA locus
   • CA mutagenic agent
   • DA codon

   (b)

   Which statement best explains the mutant cells found in culture Y?

   [1 mark]

   • AMutations can arise spontaneously during DNA replication
   • BMutations only happen when a mutagenic agent is present
   • CMutations only happen during translation
   • DMutations only happen when chromosomes fail to separate in meiosis

   (c)

   Compare the rate of mutation in the two cultures and explain the difference.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Identify the type of gene mutation shown and explain its effect on the polypeptide.

   [3 marks]

   (b)

   A different mutation is the deletion of the first base (C) of the codon CAA in the normal allele. Explain why this deletion is likely to have a greater effect on the polypeptide than a substitution that changes the codon UUC to UUU.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A genetics clinic investigates two inherited conditions. Case 1: a baby has Down's syndrome, with 47 chromosomes in each body cell, including three copies of chromosome 21. A normal human gamete contains 23 chromosomes. Case 2: a patient has sickle-cell anaemia, caused by a change in the sixth codon of the β-globin gene. This mRNA codon is GAG (glutamic acid) in the normal allele and GUG (valine) in the sickle-cell allele. GAA also codes for glutamic acid.

   (a)

   Explain how chromosome non-disjunction could have led to the baby in Case 1 having 47 chromosomes in each body cell.

   [6 marks]

   (b)

   Explain how a base substitution in the β-globin gene can change the polypeptide, as in Case 2, and why a different substitution in the same codon need not do so.

   [6 marks]

   Total for question 4: 12 marks

End of questions