90. A researcher purified a specific protein and performed structural analysis using two different techniques. The data are as follows: (1) Size-Exclusion Chromatography: Under native (non-denaturing) conditions, the protein elutes at a position corresponding to a molecular weight of 200 kDa; (2) SDS-PAGE (with β-mercaptoethanol): After boiling the sample in SDS and a reducing agent, a single sharp band appears at 50 kDa. Which of the following best describes the native structure of this protein? (A) A monomeric 200 kDa protein that is non-specifically degraded into 50 kDa fragments during electrophoresis (B) A homotetramer composed of four 50 kDa subunits held together by non-covalent interactions or disulfide bonds (C) A homodimer composed of two 100 kDa subunits that were incorrectly measured during chromatography (D) A heterotetramer composed of two 75 kDa subunits and two 25 kDa subunits. (E) A 200 kDa protein that consists of a single polypeptide chain containing four internal protease cleavage sites

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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