Question

rhodopsin is a light-sensitive receptor protein found in rod cells of the retina. rhodopsin helps transmit visual signals to the brain under low-light conditions.
the rhodopsin protein is encoded by the rho gene. the structure of the rho gene is shown in the diagram below. in the diagram, rectangles represent exons and horizontal lines represent introns.
diagram of rho gene structure with sites 1 - 4
which of the following rho gene mutations would most likely affect a person’s ability to see under low-light conditions?
choose 1 answer:
a a single base-pair insertion at site 1 that adds a “g-c” nucleotide pair to the gene sequence
b a two base-pair deletion at site 2 that removes two “a-t” nucleotide pairs from the gene sequence
c a single base-pair substitution at site 3 that changes a serine-encoding codon (agc) to an arginine-encoding codon (aga)
d a single base-pair substitution at site 4 that changes one proline-encoding codon (ccc) to another (cca)

Explanation:

1. First, recall the structure of genes: exons (rectangles) code for proteins, introns (horizontal lines) are non - coding and are removed during RNA splicing.
2. Analyze each option:

• Option A: Site 1 is an intron (horizontal line). Inserting a base - pair in an intron is less likely to affect the protein coding sequence of rhodopsin, as introns are spliced out.
• Option B: Site 2 is an intron. Deleting base - pairs in an intron is also less likely to affect the protein - coding part of the gene since introns are removed during processing.
• Option C: Site 3 is an exon (rectangle). A single base - pair substitution in an exon that changes a serine - encoding codon (AGC) to an arginine - encoding codon (AGA) will change the amino acid sequence of the rhodopsin protein. Since rhodopsin is crucial for low - light vision, changing its amino acid sequence can potentially affect its function.
• Option D: Site 4 is an exon, but the substitution changes one proline - encoding codon (CCC) to another (CCA). Since both CCC and CCA code for proline (synonymous mutation), the amino acid sequence of the rhodopsin protein remains unchanged, so it is less likely to affect the function.

Answer:

C. a single base - pair substitution at Site 3 that changes a serine - encoding codon (AGC) to an arginine - encoding codon (AGA)