23. Using RNA as a template for protein synthesis instead of translating proteins directly from the DNA is advantageous for the cell because
(A) RNA is much more stable than DNA.
(B) RNA acts as an expendable copy of the genetic material.
(C) only one mRNA molecule can be transcribed from a single gene, lowering the potential rate of gene expression.
(D) tRNA, rRNA and others are not transcribed.
(E) mRNA molecules are subject to mutation but DNA is not.

19. Nitroglycerine is a potent _____ because of the release of _____(A) antioxidant； O2 (B) vasoconstrictor； NO. (C) vasodilator； NO. (D) muscle tensor； O2 (E) All are true

20. By definition of a Lewis acid, choose the correct answer. 1. 2. 3. 4. NH35. (A) 1 and 3 are Lewis acids. (D) 1 and 4 are Lewis acids. (B) 2 and 4 are Lewis acids. (E) 2 and 3 are Lewis acids. (C) 5 is Lewis acid.

21. What enzyme breaks apart the hydrogen bonds between two strands of DNA? (A) Polymerase I. (B) Polymerase III. (C) Ligase. (D) Helicase. (E) Okazaki.

22. Which statement about the elongation of the lagging strand during DNA replication is correct? (A) It is synthesized in a 3' 5' direction. (C) It requires a short RNA primer to proceed. (B) It progresses (grows) toward the replication fork. (D) It is synthesized by DNA ligase. (E) It is synthesized continuously.

24. Which of the following is   not   true of RNA processing? (A) Exons are cut out before mRNA leaves the nucleus. (B) Nucleotides may be added at both ends of the RNA. (C) Ribozymes may function in RNA splicing. (D) RNA splicing can be catalyzed by spliceosomes. (E) A primary transcript is often much longer than the final RNA molecule that leaves the nucleus.

25. Which of the following is most likely to have a small protein called ubiquitin attached to it? (A) A regulatory protein that requires sugar residues to be attached. (B) A cell surface protein that requires transport from the ER. (C) An mRNA that is leaving the nucleus to be translated. (D) A cyclin that usually acts in G1, now that the cell is in G2. (E) An mRNA produced by an egg cell that will be retained until after fertilization. 

第26至50題每題2.5分 26. Sam is trying to separate a protein mixture containing three proteins A, B, C. Protein A: homotetramer, Mr = 120,000, pI =6； Protein B: monomer, Mr = 80,000, pI=7； Protein C: monomer, Mr = 13,000, pI=9. Please choose the correct one from the following descriptions: (A) If Sam dissolves the mixture in 0.1 M Tris-HCl (pH 7) and uses SDS-PAGE to separate the mixture, the protein bands in the gel from top to bottom should be B-A-C. (B) If Sam dissolves the protein mixture in 20 mM phosphate buffer (pH 8) and loads it to an anion exchange column equilibrated with the same buffer, only protein C can bind to the column. (C) If Sam dissolves the mixture in 0.1 M Tris-HCl (pH 7) and uses size exclusion column to separate the mixture, protein C will be first eluted. (D) If Sam dissolves the mixture in 0.1 M Tris-HCl (pH 7) and uses size exclusion column to separate the mixture, the elution sequence is B-A-C. (E) If Sam dissolves the protein mixture in 20 mM phosphate buffer (pH 7) and loads it to a cation exchange column equilibrated with the same buffer, only protein A can bind to the column.

27. Which description is correct? (A) All the residues in the peptide with sequence "AKLFPIEARQA" have only one chiral center. (B) The peptide with sequence “AKQHPMLWNR” does not contain the residue which can be O-glycosylated. (C) The pI value of peptide with sequence "DSQWMKLGIV" should be near 9. (D) When the peptide with sequence "CYTTLNKTG" was completely digested by trypsin, we can get one tripeptide and one hexapeptide. (E) The peptide with sequence "AKLFPIEARGQA" does not contain any aromatic ring.

28. What are the expected changes in kinetics in the presence of a competitive inhibitor? (A) decreases, appears to decrease. (B) does not change, appears to decrease. (C) decreases, appears to increase. (D) does not change, appears to increase. (E) decreases, does not change.

29. Which of the following explains why enzymes are extremely effective catalysts? (A) An enzyme stabilizes the transition state. (B) Enzymes bind very tightly to substrates. (C) Enzymes release products very rapidly. (D) An enzyme can convert a normally endergonic reaction into an exergonic reaction. (E) An enzyme lowers the energy of activation only for the forward reaction.