83. Human somatic cells in culture stop dividing after approximately 50 to 60 population doublings, known as the Hayflick limit. This replicative senescence is associated with the progressive shortening of the ends of chromosomes. Which enzymatic activity is normally suppressed in most somatic cells but reactivated in cancer cells and could prevent this shortening? (A) DNA ligase (B) Primase (C) Topoisomerase (D) 3' to 5' exonuclease activity of DNA polymerase (E) Reverse transcriptase activity of telomerase

83. Human somatic cells in culture stop dividing after approximately 50 to 60 population doublings, known as the Hayflick limit. This replicative senescence is associated with the progressive shortening of the ends of chromosomes. Which enzymatic activity is normally suppressed in most somatic cells but reactivated in cancer cells and could prevent this shortening?
(A) DNA ligase
(B) Primase
(C) Topoisomerase
(D) 3' to 5' exonuclease activity of DNA polymerase
(E) Reverse transcriptase activity of telomerase

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統計： A(0), B(0), C(1), D(0), E(3) #3853388

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「海佛烈克極限 (Hayflick li...

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13. Suppose a person begins breathing very rapidly while resting (hyperventilating), which of the following best describes how homeostasis is restored? (A) Rapid breathing decreases blood CO2 levels, thereby raising blood pH. The frontal cortex detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate. (B) Rapid breathing increases blood CO2 levels, thereby causing the blood pH to drop. The hypothalamus detects the change and decreases stimulation of the rib muscles and the diaphragm, thereby reducing the breathing rate. (C) Rapid breathing decreases blood CO2 levels, thereby raising blood pH. The hypothalamus detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate. (D) Rapid breathing increases blood CO2 levels, thereby causing the blood pH to drop. The medulla oblongata detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate. (E) Rapid breathing decreases blood CO2 levels, thereby raising blood pH. The medulla oblongata detects the change and reduces stimulation of the rib muscles and the diaphragm, thereby decreasing the breathing rate.

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