Gene mutationsEdexcel International A Level Biology: Subtopic test

1. 1
   In sickle cell anaemia, a change in the gene for the beta chain of haemoglobin alters one codon in the mRNA from GAG, which codes for glutamic acid, to GUG, which codes for valine. All the other codons are unchanged.

   (a)

   What type of gene mutation has occurred?

   [1 mark]

   • ADeletion
   • BInsertion
   • CSubstitution
   • DDuplication

   (b)

   What is the effect of this mutation on the polypeptide?

   [1 mark]

   • AOne amino acid is replaced by a different amino acid
   • BEvery amino acid after the mutation is changed
   • CThe polypeptide is shortened because translation stops early
   • DThere is no change to the polypeptide

   (c)

   Explain how this change in one codon can alter the properties of the haemoglobin protein.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The mRNA of a gene has the codon sequence AUG GCA UUC GAA UAA. A mutation deletes the first base of the second codon, the G of GCA, and the rest of the mRNA is unchanged.

   (a)

   Which type of mutation is this?

   [1 mark]

   • ASubstitution
   • BInsertion
   • CDuplication
   • DDeletion

   (b)

   What is the consequence of this mutation for the codons that follow it?

   [1 mark]

   • AOnly the second codon is changed
   • BEvery codon from the deletion onwards is altered because the reading frame shifts
   • CNo codons change because the genetic code is degenerate
   • DOnly the stop codon is changed

   (c)

   Explain why deleting three consecutive bases from the middle of a gene may be less harmful than deleting a single base.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In an mRNA, the codon GAA codes for glutamic acid. The codon GAG also codes for glutamic acid, the codon GAU codes for aspartic acid and the codon UAA is a stop codon.

   (a)

   Describe and explain the effect on the polypeptide of a substitution that changes the codon GAA to (i) GAG, (ii) GAU and (iii) UAA.

   [3 marks]

   (b)

   Explain why many gene mutations have no observable effect on the phenotype.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Researchers studying a type of skin cancer find that the tumour cells carry a mutation in a gene whose protein normally helps to control cell division. The mutation arose when a skin cell replicated its DNA before dividing.

   (a)

   Explain how an error during DNA replication can result in a substitution, an insertion or a deletion, and compare the likely effects of these mutations on a polypeptide.

   [6 marks]

   (b)

   Explain how mutations can lead to cancer, and why most mutations in body cells do not.

   [6 marks]

   Total for question 4: 12 marks

End of questions