20. Supercoiling of DNA:
(A) is not observed in prokaryotes.
(B) requires the action of topoisomerase enzymes.
(C) does not require ATP.
(D) is not observed in eukaryotes.
(E) None of the above is true.

16. Some hormones are derived from amino acids； for example, catecholamines are derived from______ while serotonin is derived from______ . (A) tyrosine； tryptophan (B) tryptophan； lysine (C) tyrosine； histidine (D) tryptophan； arginine (E) histidine； lysine

17. Which of the following is not true during fasting? (A) During fasting, the liver maintains blood glucose levels. (B) Amino acids from muscle protein are a major carbon source for the production of glucose by the pathway of gluconeogenesis. (C) As amino acid carbons are converted to glucose, the nitrogens are converted to urea. Thus, the urinary excretion of urea is high during fasting. (D) As fasting progresses, however, the brain begins to use ketone bodies, sparing blood glucose. (E) Only muscle proteins are mobilized for energy requirement.

18. Which of the following is not involved in triggering insulin from beta-cells release in response to glucose? (A) Reduced eflux through ATP-gated K*-channels (B) Increased concentrations of ATP (C) cAMP-dependent phosphorylation of lipase (D) Phosphorylation of glucose to glucose-6-phosphatc (E) Increased Cazt levels

19. Which contain(s) a phosphoanhydride bond? (A) DNA(B)RNA (C) protein (D) ATP (E) Both DNA and RNA

21. What is the main damaging effect of UV radiation on DNA? (A) Depurination (B) Deamination(C)Formation of thymine dimers (D) Single strand break (E) Dehydration

22. The binding of lac repressor to DNA could be considered to be analogous to: (A) competitive inhibition of an enzyme. (B) mixed-type inhibition of an enzyme. (C) uncompetitive inhibition of an enzyme. (D) allosteric effects in enzyme regulation. (E) None of the above

23. Protein translation begins: (A) at the replication fork.(B)on the(C)at the start codon.  (D) in nucleus. (E) All of these

24. Eukaryotic mRNA binding to the ribosomes is facilitated by_______. (A) the Shine Dalgarno sequence (B) the 7-methyl guanosine cap (C)tRNA(D) snRNA (E) All of the above

25. Cellulose fibers resemble with the protein structure in the form of: (A) β-sheets. (B) α-helices. (C) β-turns. (D)AII of the above(E) both β-sheets and u-helices. 

26. Which description is correct? (A) Organisms which could use light as energy source are classified as autotrophs. (B) Microtubules and intermediate filaments help to produce motion of organelles or of the whole cell. (C) Isoleucine has four stereoisomers. (D) H.O in liquid water forms more hydrogen bonds than in ice. (E) I M NaCl and 1 M H.SO, has the same osmolarity.