25. Diastereomers are:
(A) mirror-image, nonsuperimposable stereoisomers.
(B) non-mirror-image, nonsuperimposable stereoisomers.
(C) mirror-image, superimposable stereoisomers.
(D) stereoisomers with one or more double bonds.
(E) None of the above.

21. A mutation in one gene that counteracts the effects of a mutation in another gene is known as a: (A) temperature-sensitive mutation. (B) recessive mutation. (C) suppressor mutation. (D) conditional mutation. (E) None of these.

22. In RNA interference studies, the double-stranded RNA: (A) disrupts the target DNA sequence. (B) results in the destruction of the target mRNA. (C) destroys the target protein. (D) All of the above. (E) None of the above.

23. Which of the following is a typical feature of prokaryotic genes? (A) Polycistronic messenger RNAs. (B) Complex transcription units. (C) Introns. (D) A and C. (E) None of the above.

24. Splice sites in pre-mRNA are marked by two universally conserved sequences contained: (A) in the middle of the intron. (B) at the ends of the exons. (C) at the ends of the introns. (D) in the middle of the exon. (E) None of the above.

26. Please choose the     correct     answer from the following descriptions: (1) The non-amino acid part of a conjugated protein is called its prosthetic group. (2) a-keratin is a protein in which the polypeptides are mainly in the a-helix conformation. (3) Arg side-chain is an imidazole group and has important role in maintaining pH in blood. (4) Every turn of a-helix is 10 A in length. (5) Phenylisothiocyanate could only be used to determine the first amino acid of a protein. (A) 1 and 2 are correct. (B) 1, 4, and 5 are incorrect. (C) 1, 3, and 5 are incorrect. (D) 2 and 5 are correct. (E) 3 and 4 are correct.

27. Please choose the     correct     answer from the following descriptions: (1) All amino acids in nature are L-amino acids. (2) The pl value of the peptide "GAILLSDN" is lower than that of the peptide "VPRYTTN". (3) Paralogous genes are presumed to have been derived by gene duplication followed by gradual changes in the sequences of both copies. (4) Collagen is full of a-helix structure and has Gly-X-Pro repeating sequence. (5) Trypsin could hydrolyze the "X-K" peptide bond. (A) 1, 3, and 5 are correct. (B) 2 and 5 are correct. (C) 3, 4, and 5 are correct. (D) Only 3 is correct. (E) 1, 4, and 5 are incorrect.

28. Please choose the    correct    answer from the following descriptions: (1) For naming the RS system, -CH2OH group has higher priority than -CHO group.  (2) Cyanogen bromide could be used to cleave the "X-C" peptide bond. (3) The role of glycogen is energy storage in bacteria and animal cells. (4) The structure of lactose is  (5) Within 20 common amino acids, Thr and Leu are the only two amino acids with two chiral centers. (A) 2 and 4 are     incorrect    . (B) 3 and 4 are correct. (C) 1, 3, and 5 are correct. (D) 3 and 5 are correct. (E) 1, 2, and 4 are     incorrect    .

29. Which of the following pairs of bonds within a peptide backbone show free rotation around both bonds? (A) Ca-C and N-Ca. (B) C=O and N—С. (C) C=O and N-Ca. (D) N-C and Ca-C. (E) N-Ca and N—C.

30. All of the following contribute to the large, negative, free-energy change upon hydrolysis of "high-energy" compounds     except    : (A) electrostatic repulsion in the reactant. (B) low activation energy of forward reaction. (C) stabilization of products by extra resonance forms. (D) stabilization of products by ionization. (E) stabilization of products by solvation.

31. In a plot of I/V against 1/[S] for an enzyme-catalyzed reaction, the presence of a competitive inhibitor will alter the: (A) curvature of the plot. (B) intercept on the 1/[S] axis. (C) intercept on the I/V axis. (D) pK of the plot. (E)