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(D) Efflux of ATP
(E) Influx of Na
+
ions
84. Which of the following most likely explains
repolarization?
(A) Na
+
–K
+
–ATPase ceases to function
(B) Rapid influx of Na
+
ions
(C) Rapid efflux of K
+
ions
(D) Rapid influx of Cl
–
ions
(E) Rapid efflux of Cl
–
ions
Questions 81–84 refer to the action potential of a nerve fiber. The diagram below demonstrates the effect
of several stimuli on the membrane potential of a neuron. Each triangle indicates the time at which a
stimulus is applied to the nerve.
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Lesson 9
264
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ARCO
■ SAT II Subject Tests
Questions 85–87
Experiments have demonstrated that DNA is a very well organized, cooperative structure. The two
strands of the DNA double helix are joined by hydrogen bonds between the paired bases. Adenine and
thymine are held together by two hydrogen bonds, while three hydrogen bonds will form between and
join the bases cytosine and guanine. Five DNA melting curves, labeled A, B, C, D, and E, are illus-
trated below. The melting point of a given DNA molecule is described as being the temperature at
which half the double helix is unwound. Absorption, as measured by the spectrophotometer, increases
as the DNA melts. Once completely melted, the curve levels.
85. Which species of DNA melts at the lowest
temperature?
(A) A
(B) B
(C) C
(D) D
(E) E
86. Which species of DNA possesses the greatest
content of adenine-thymine base pairs?