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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

89. The figure below shows the absorbance curve at 260 nm of five 100 bp double-stranded DNA fragments (samples A to E) when subjected to heat. DNA denaturation produces a hyperchromic effect. Which sample has the highest G-C content, according to this graph? (A) A (B) B (C) C (D) D (E) E

88. Glucose-6-phosphatase is a key regulatory enzyme localized in the endoplasmic reticulum membrane. In von Gierke disease, the deficiency of this enzyme prevents the hydrolysis of glucose-6-phosphate. Which of the following biochemical pathways is/are directly impaired by this deficiency during fasting? (A) Glycogenesis only (B) Glycogenolysis only (C) Gluconeogenesis only (D) Both glycogenolysis and gluconeogenesis (E) The pentose phosphate pathway

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