The LD50 of Vibrio cholerae is 108 cells through the oral route. If the bacterial cells are ingested with bicarbonate, the LD50 drops to 104. Which of these explanations is the most likely?
A) Sodium bicarbonate decreases the virulence of Vibrio cholerae.
B) Stomach acid decreases the virulence of Vibrio cholerae.
C) Vibrio cholerae makes toxin only in the presence of stomach acid.
D) Stomach acid increases the virulence of Vibrio cholerae.
E) Sodium bicarbonate inactivates Vibrio cholerae.
Ans: D) Stomach acid increases the virulence of Vibrio cholerae.
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A. It introduces single-strand breaks in the chromosome. B. It degrades the 5'-ending strand to create 3'-ending single-strand overhangs. C. It is a helicase that assists in separating the DNA strands at the site of breakage. D. It is a single-strand DNA binding protein that coats the 3'-ending single-strand overhang. E. It is a RecA-class recombinase that mediates strand invasion.
Photolyases are present in all cells except in:
A. bacteria. B. archaea. C. humans. D. yeast. E. both C and D.
Disruptive selection favors
a. the intermediate forms. b. an extreme at one end of a range of variation. c. the extremes at both ends of a range of variation. d. a greater number of individuals in the center of the bell-shaped curve. e. neutral traits.
Every amino acid contains
A) an amino group and a hydroxyl group. B) an amino group and an acetyl group. C) a hydroxyl group and a carboxyl group. D) a carboxyl group and an amino group. E) a carboxyl group and a hydroxyl group.