MutationsEdexcel A-Level Biology A: Subtopic test

1. 1
   The coding strand of part of a gene has the base sequence CAG TTA GGC. After an error during DNA replication, the same part of the gene in one daughter cell has the base sequence CAG TAA GGC. The mRNA has the same sequence as the coding strand, with uracil in place of thymine, and UAA is a stop codon.

   (a)

   What type of change has occurred in the gene?

   [1 mark]

   • AA substitution of one base
   • BA deletion of one base
   • CAn insertion of one base
   • DA deletion of one triplet

   (b)

   What is the effect of this change on the polypeptide?

   [1 mark]

   • AOne amino acid is replaced by a different amino acid and the rest are unchanged
   • BThere is no change, because the genetic code is degenerate
   • CAll the amino acids after the change are different
   • DThe polypeptide is shorter, because a stop codon is introduced

   (c)

   Explain how an error in DNA replication could produce this change in the base sequence.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The coding strand of the start of a gene has the base sequence ATG GAA CTT AGC. In one person a mutation deletes a single base, the G at the start of the second triplet, so the sequence reads ATG AAC TTA GC.

   (a)

   What type of mutation has occurred?

   [1 mark]

   • AA substitution
   • BAn insertion
   • CA deletion that causes a frameshift
   • DA silent mutation

   (b)

   Why does the deletion of one base usually have a greater effect than the substitution of one base?

   [1 mark]

   • AA deletion always creates a stop codon
   • BAll the codons after the deletion are changed, because the bases are read in different groups of three
   • CA deletion is the only mutation that changes the primary structure
   • DA deletion removes the start codon in every case

   (c)

   Explain how this mutation could change the structure of the protein made from this gene.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cystic fibrosis is caused by mutations in the CFTR gene, which codes for a protein that forms a channel for chloride ions in the cell-surface membranes of epithelial cells. The most common mutation is a deletion of three bases, which removes one amino acid, phenylalanine, from the CFTR protein. The altered protein is folded incorrectly and does not reach the cell-surface membrane.

   (a)

   Explain how the deletion of three bases in the CFTR gene leads to a protein that cannot function as a chloride ion channel.

   [3 marks]

   (b)

   Explain how the absence of working CFTR channels in the cell-surface membranes of cells lining the airways causes thick, sticky mucus.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Cystic fibrosis is caused by mutations in the CFTR gene, which codes for a chloride ion channel in cell-surface membranes. Many different mutations of the CFTR gene are known to cause cystic fibrosis, including substitutions of one base for another, deletions and insertions of bases, and substitutions that create a stop codon. Some single-base substitutions in the gene cause severe disease, some cause milder disease and some have no effect.

   (a)

   Describe how an error during DNA replication can lead to a mutation in the CFTR gene, and explain how such a mutation can result in a CFTR protein that does not work.

   [6 marks]

   (b)

   Explain why single-base substitutions in the CFTR gene can have no effect, a mild effect or a severe effect.

   [6 marks]

   Total for question 4: 12 marks

End of questions