Question

Percent sequence divergences between two sister species were calculated for an exon (E), an intron (I), and the flanking neutral (N) region of a gene that is under purifying selection. Which one of the following options best describes the expected divergence in E, I, and N between the sister species?

• A. E > I > N
• B. E > I = N
• C. E < I = N
• D. E = I > N

Hint:

Purifying selection removes mutations in the coding exon that change the protein, while introns and flanking neutral regions build up mutations at a similar, unconstrained rate.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We compare three parts of the same gene, an exon (E, the coding part), an intron (I, the non-coding part spliced out), and a flanking neutral region (N, DNA just outside the gene with no known function), and rank how much each has diverged between two sister species.

Step 2: Key Concept:
Purifying (negative) selection removes mutations that damage protein function. Since the exon's sequence directly codes for protein, most mutations there are harmful and get weeded out, keeping the exon's sequence very similar between the two species, that is, low divergence. Introns and flanking neutral regions do not code for protein, so mutations there are mostly neutral, and neutral mutations accumulate at close to the background mutation rate, with no strong selection removing them.

Step 3: Detailed Explanation:
Because the exon is under strong purifying selection, its divergence (E) stays low compared to regions with no such constraint. Because introns and flanking neutral DNA are both largely free of selection, they should accumulate mutations at roughly the same, higher rate, so their divergence values, I and N, should be close to each other and both higher than E. This rules out E > I > N and E > I = N, since both wrongly put E on top, and E = I > N, since it wrongly puts N as the lowest and treats E as equal to I.

Step 4: Final Answer:
The exon shows the least divergence, while the intron and the neutral flanking region, both free of purifying selection, show similar and greater divergence.
\[ \boxed{E \lt I = N} \]