MutationsEdexcel A-Level Biology A: Mind map

Exam questions on Mutations

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.
   Explain how an error in DNA replication could produce this change in the base sequence.2 marks
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.
   Explain how this mutation could change the structure of the protein made from this gene.2 marks
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.
   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