Question

avpr2 is a receptor protein that helps maintain water homeostasis in humans. the binding of avpr2 to antidiuretic hormone in the kidney promotes water retention, helping to prevent dehydration and electrolyte imbalance. the avpr2 protein is encoded by the avpr2 gene. the structure of the avpr2 gene is shown below. in the diagram, rectangles represent exons and horizontal lines represent introns. diagram of avpr2 gene with site 1, site 2, site 3, site 4 indicated which of the following avpr2 gene mutations would most likely affect a person’s ability to properly maintain water homeostasis? choose 1 answer: a a single base - pair substitution at site 1 that changes one lysine - encoding codon (aaa) to another (aag) b a single base - pair insertion at site 2 that shifts the reading frame for subsequent codons c a single base - pair deletion at site 3 that removes an “a - t” nucleotide pair from the gene sequence d a single base - pair substitution at site 4 that results in an early stop codon

Explanation:

1. Analyze Option A: A base - pair substitution from AAA to AAG still codes for lysine (since both are lysine - encoding codons). So, the amino acid sequence of the AVPR2 protein will not be changed significantly, and the protein's function is likely to remain intact. Thus, this mutation is not likely to affect water homeostasis.
2. Analyze Option B: Site 2 is in an exon (the black - colored region in the gene diagram, exons are the coding regions). A single base - pair insertion at Site 2 will shift the reading frame of the subsequent codons. This frameshift mutation will lead to a completely different amino acid sequence being translated from the mutated gene. Since the AVPR2 protein's structure and function are dependent on its correct amino acid sequence, a frameshift mutation in an exon (coding region) will likely result in a non - functional or malfunctioning AVPR2 protein. A non - functional AVPR2 protein will not be able to bind to the antidiuretic hormone properly, and thus will affect the ability to maintain water homeostasis.
3. Analyze Option C: Site 3 is in an intron (the horizontal line between exons, introns are non - coding regions). Introns are removed during RNA splicing, so a base - pair deletion in an intron is not likely to affect the amino acid sequence of the AVPR2 protein. Therefore, this mutation will not affect the protein's function related to water homeostasis.
4. Analyze Option D: Site 4 is in an exon, but a single base - pair substitution that results in an early stop codon. However, we need to consider the position of Site 4. If the stop codon is near the end of the coding sequence, the resulting protein might still have most of its functional domains. But a frameshift mutation (as in Option B) in the middle of an exon (coding region) is more likely to completely disrupt the protein's structure and function compared to a single base - pair substitution that causes an early stop, especially if the stop is not too early in the coding sequence. Also, from the gene diagram, Site 2 is in a relatively long exon, so a frameshift there will have a more profound effect on the protein sequence.

Answer:

B. a single base - pair insertion at Site 2 that shifts the reading frame for subsequent codons