49. A sequence of amino acids in a certain protein is found to be –Ser–Gly–Pro–Gly–. The sequence is most probably part of a(n)______.
(A) β-turn
(B) parallel β-sheet
(C) α-helix
(D) α-sheet
(E) antiparallel β-sheet

45. An analysis of chromosomal DNA, using the Southern blot technique, involves the following five major steps: ①Autoradiography； ②Blotting； ③ Cleavage；④Electrophoresis； ⑤Hybridization；Which of the following sequences of steps best illustrates this technique? (A)①②③④⑤ (B) ①③②④⑤ (C) ③⑤②④①(D) ③②⑤④①(E) ③④②⑤①

46. The two nitrogen atoms in urea arise from______. (A) ammonia and glutamine (B) glutamine and aspartic acid (C) glutamine and glutamic acid (D) glutamic acid and alanine (E) ammonia and aspartic acid

47. Which of the following statements about antibiotic inhibitors of protein synthesis is correct? (A) Chloamphenicol inhibits the peptidyl transferase activity of the large ribosomal subunit in eukaryotes. (B) Cycloheximide inhibits the peptidyl transferase activity of the large ribosomal subunit in eukaryotes. (C) Puromycin blocks protein synthesis in prokaryotes only. (D) Streptomycin inhibits protein synthesis by binding to 30S ribosomal subunits. (E) Erythromycin inhibits translocation in eukaryotes only. 

48. A biochemist is attempting to separate a protein X from protein C in a solution. The proteins have the following properties: What type of protein separation techniques might be the best one to separate protein X from protein C? (A) Gel filtration (B) SDS-polyacrylamide gel electrophoresis (C) Affinity chromatography (D) Ion-exchanger chromatography (E) Agarose gel electrophoresis

50. Which of the following is (are) true of the oxidation of 1 mol of palmitate (a 16-carbon saturated fatty acid； 16:0) by the β-oxidation pathway, beginning with the free fatty acid in the cytoplasm?①Activation of the free fatty acid requires the equivalent of two ATPs.② Inorganic pyrophosphate (PPi) is produced.③Carnitine functions as an electron acceptor.④8 mol of FADH2 are formed.⑤8 mol of acetyl-CoA are formed.⑥There is no direct involvement of NAD+ . (A)①and⑤ (B) ①②and⑤(C) ①② and⑥ (D) ①③ and ⑤ (E)④ and⑤

51. Which of the following peptides is more likely to be nuclear targeting signal? (A) ser-ser-thr-asn-arg-lys-met-thr (B) arg-lys-lys-arg-arg-lys-leu-met (C) met-leu-ile-phe-leu-met-ile-ile (D) ser-ser-thr-thr-thr-thr-ser-thr-ser (E) asn-lys-ser-asn-lys-ser-asn-lys-ser

52. Myoglobin and the subunits of hemoglobin have______ . (A) no obvious structural relationship (B) very different primary and tertiary structures (C) very similar primary and tertiary structures (D) very similar primary structures, but different tertiary structures (E) very similar tertiary structures, but different primary structures

53. Which one of the following analytical techniques does not help illuminate a gene’s cellular function? (A) Southern blotting (B) DNA microarray analysis (C) Protein chip analysis (D)Two-dimensional gel electrophoresis (E) Two-hybrid analysis

54. Hormone-activated phospholipase C can convert phosphatidylinositol 4,5-bisphosphate to______ . (A) diacylglycerol + inositol triphosphate (B) diacylglycerol + inositol+ phosphate (C) glycerol + inositol + phosphate (D) glycerol + phosphoserine (E) phosphatidyl glycerol + inositol + phosphate

55. Oxidative phosphorylation and photophosphorylation share all of the following except______  . (A) chlorophyll (B) involvement of cytochromes (C) participation of quinones (D) proton pumping across a membrane to create electrochemical potential (E) use of iron-sulfur proteins