2024HSCShort answer

Q28 (b)

4 marks

Question

Cystic fibrosis is an inherited disorder that causes damage to the lungs, digestive system and other organs in the body. A person with cystic fibrosis will have two faulty recessive alleles for the cystic fibrosis gene (CFTR) on chromosome 7.
Explain how the deletion of nucleotides in the CFTR gene removes only one amino acid. Include reference to the nucleotides that code for isoleucine and phenylalanine amino acids.

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Solution

Criteria	Marks
Provides a thorough explanation of how the deletion removes only one amino acid; refers to isoleucine, phenylalanine and the final amino acid sequence	4
Provides a sound explanation of the deletion of one amino acid; refers to appropriate amino acids or the effects of deletions	3
Demonstrates some understanding of how mRNA codes for amino acids or of the effects of deletions	2
Provides some relevant information	1

Sample answer:

A triplet of mRNA nucleotides codes for one amino acid, however, the three nucleotides that are removed span across two triplet codons – the codon for isoleucine and the codon for phenylalanine. Therefore, it would be expected that both amino acids are affected. However, isoleucine can be coded for by a number of different triplet nucleotide combinations. The correct coding is AUC, but after the deletion of the three nucleotides, the code left is AUU, which also codes for isoleucine. Thus, only the phenylalanine amino acid is removed.

Supplied marking guidance · human reviewed

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Syllabus alignment

Module 6 · Genetic ChangeModule 5 · Heredity

MutationCompare causes, processes and effects of point and chromosomal mutations.
DNA and Polypeptide SynthesisModel transcription and translation, including the roles of mRNA and tRNA.

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