20. In the activated-methyl cycle for the synthesis of methionin and S-adenosylmethionine (adoMet), the methyl donor for the regeneration of methionin from homocystein is:
(A) S-adenosylmethionine.
(B) N⁵-methyltetrahydrofolate.
(C) N⁵-formyltetrahydrofolate.
(D) glutathione.
(E) -methylenetetrahydrofolate.

16. The enzyme that catalyzes the conversion of GMP to GDP is: (A) nucleoside diphosphate kinase. (B) ribonucleotide reductase. (C) adenylate kinase. (D) guanine phosphoribosyltransferase. (E) nucleoside monophosphate kinase.

17. Which of the following descriptions about human hormones is     false    ? (A) Prostaglandins are examples of eicosanoid hormones. (B) The tropic hormones (such as thyrotropin, somatotropin, and luteinizing hormone) are produced and released by the anterior pituitary. (C) NO is derived from amino acid asparagine. (D) Eicosanoids are paracrine hormones. (E) Catecholamines are derived from amino acid tyrosine.

18. Which of the following descriptions is     false    ? (A) The double helix of DNA in the B-form is stabilized by nonspecific base-stacking interaction between two adjacent bases in the same strand. (B) The phosphodiester bond that joins adjacent nucleotides in DNA associates ionically with metal ions, polyamines, and proteins. (C) The phosphodiester bond that joins adjacent nucleotides in DNA is susceptible to alkaline hydrolysis. (D) The pentoses in nucleic acids do     not     undergo interconversion of the straight-chain form and the ring form. (E) The size and base composition of a double-stranded DNA are factors that affect the 'melting temperature of DNA.

19. In liver mitochondria, catalyzes the reaction to release the amino group from glutamine; the products are glutamate and ammonia. (A) glutaminase. (B) glutamate dehydrogenase. (C) glutamate aminotransferase. (D) glutamine aminotransferase. (E) glutamine amidotransferase.

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.

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.

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.