Questions: Protein Evolution and Functional Constraint

Question 1

A researcher computes dN/dS = 0.04 for histone H4 and dN/dS = 0.95 for fibrinopeptide A. What does this contrast indicate?

• A. Histone H4 is under positive selection; fibrinopeptide A is under strong purifying selection
• B. Histone H4 is under strong purifying selection; fibrinopeptide A evolves nearly neutrally with little functional constraint
• C. Both proteins are under equivalent levels of purifying selection, just operating in different directions
• D. The dN/dS ratio is not comparable across proteins from different functional categories

Answer: (B)

Low dN/dS (histones at 0.04) means nearly all amino acid changes are being eliminated by purifying selection — almost every nonsynonymous mutation damages function. High dN/dS approaching 1 (fibrinopeptides at 0.95) means amino acid changes accumulate at nearly the neutral rate — functional constraint is minimal. Histones must maintain precise contacts with DNA and other histones; fibrinopeptides are cleaved off during blood clotting and serve little ongoing functional role.

Question 2

Within a single enzyme, active-site residues typically show much lower dN/dS than surface-exposed loops. What explains this within-protein variation?

• A. Active-site residues mutate more frequently because substrates induce localized DNA damage
• B. Surface loops are shielded from selection by being buried within the protein fold
• C. Active-site residues perform precise catalytic and binding functions where most amino acid changes are lethal; surface loops tolerate substitutions with little effect on function
• D. Active-site residues have lower mutation rates due to proximity to DNA repair machinery

Answer: (C)

dN/dS reflects functional constraint at specific positions within a protein. Active-site residues are under intense purifying selection — a change in charge, size, or chemistry at a catalytic residue can destroy enzymatic activity entirely. Surface loops often tolerate even non-conservative substitutions because they lack specific functional requirements. This within-protein variation makes dN/dS analysis powerful for identifying functionally important sites.

Question 3

A gene with dN/dS = 0.1 is evolving slowly because selection on that gene is relaxed or absent.

• T. True
• F. False

Answer: False

This reverses the inference. dN/dS = 0.1 means strong purifying selection is actively removing most nonsynonymous mutations — the protein is under tight functional constraint. The low ratio does not mean evolution is 'inactive'; it means selection is working hard to suppress amino acid change. Relaxed or absent selection would produce dN/dS approaching 1 (neutral evolution), not a low ratio.

Question 4

Finding dN/dS > 1 for a gene is evidence that natural selection has been favoring amino acid changes in that gene — a signature of positive selection.

• T. True
• F. False

Answer: True

dN/dS > 1 means nonsynonymous substitutions accumulate faster than the neutral rate. Since most amino acid changes are deleterious and purged by selection, rates exceeding neutral expectation can only result from natural selection actively favoring new amino acid variants — positive selection. This signature appears in immune genes under pathogen-driven arms races, reproductive proteins under sexual selection, and genes adapting to novel environments.

Question 5

What does the dN/dS ratio measure, and why is the synonymous substitution rate (dS) used as the denominator rather than some absolute mutation rate?

Model answer: dN/dS compares the rate of amino acid-changing (nonsynonymous) substitutions to the rate of silent (synonymous) substitutions. Synonymous sites are used as the denominator because they mutate at approximately the neutral rate — codon redundancy means synonymous changes usually don't alter the protein, so most escape selection. This makes dS a built-in control for the background mutation rate of that genomic region. By expressing dN relative to this neutral baseline, we can determine whether amino acid changes are accumulating faster than neutral (positive selection, dN/dS > 1), at the neutral rate (dN/dS ≈ 1), or being actively suppressed (purifying selection, dN/dS < 1).

The elegance of this metric is that it is self-normalizing: by comparing two substitution rates in the same sequence, it controls for differences in mutation rate across genes, chromosomal regions, and taxa. This makes dN/dS broadly applicable for comparing selective pressures across the tree of life.