Mutations and their effectsAQA A-Level Biology: Subtopic test

1. 1
   A short section of the coding strand of a gene reads ATG GAA CTT AAG GGC and codes for the amino acids methionine, glutamic acid, leucine, lysine and glycine. A mutation changes the third base of the second triplet from A to G, giving GAG, which also codes for glutamic acid. A different mutation changes the same triplet from GAA to TAA.

   (a)

   What type of gene mutation changed GAA to GAG?

   [1 mark]

   • ADeletion
   • BInversion
   • CAddition
   • DSubstitution

   (b)

   Why does the change from GAA to GAG not alter the polypeptide?

   [1 mark]

   • AThe genetic code is non-overlapping
   • BThe DNA is not transcribed
   • CEach amino acid is coded for by only one triplet
   • DThe genetic code is degenerate, so more than one triplet can code for the same amino acid

   (c)

   Explain the effect on the polypeptide of the mutation that changes GAA to TAA.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The coding strand of part of a gene reads CAT GAT TCC AAG and codes for four amino acids. A mutation deletes the first G of the second triplet.

   (a)

   Which type of mutation is this?

   [1 mark]

   • ADeletion
   • BSubstitution
   • CInversion
   • DTranslocation

   (b)

   Which statement about the polypeptide is correct?

   [1 mark]

   • AOnly the second amino acid is changed
   • BThe polypeptide is unchanged because the genetic code is degenerate
   • CEvery amino acid from the second onwards may be changed
   • DOnly the last amino acid is changed

   (c)

   Explain why a deletion of one base usually has a greater effect on a polypeptide than a substitution of one base.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A researcher grew a culture of bacteria and counted the number of cells that had mutated to become resistant to an antibiotic. In an untreated culture there were 3 resistant cells per million cells. In a culture exposed to ultraviolet light there were 450 resistant cells per million cells. The resistance was caused by a single-base change in a gene coding for a bacterial protein to which the antibiotic normally binds.

   (a)

   Explain the results of the researcher's experiment.

   [3 marks]

   (b)

   Explain how a single-base substitution in the gene could result in a bacterium becoming resistant to the antibiotic.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The CFTR gene codes for a membrane protein. The most common mutation causing cystic fibrosis is the deletion of three bases from the gene, which removes one amino acid from the protein. Other mutations of the CFTR gene include a substitution that creates a premature stop codon and the addition of one base.

   (a)

   Compare the likely effects on the CFTR polypeptide of the three types of mutation: the deletion of three bases, the substitution that creates a stop codon, and the addition of one base.

   [6 marks]

   (b)

   Explain why the number of mutations in the CFTR gene increases on exposure to a mutagenic agent, and why a single-base substitution in the gene may have no effect, a small effect or a major effect on the protein.

   [6 marks]

   Total for question 4: 12 marks

End of questions