20. The function of DNA topoisomerases is:
(A) packaging DNA into nucleosomes.
(B) forming cruciform DNA.
(C) unwinding G:C rich areas in DNA.
(D) breaking one or more strands of DNA, winding them tighter or looser, and rejoining the ends.
(E) promoting DNA hybridization.

16. Which one has a less relevance to cardiovascular diseases? (A) High LDL. (B) High HDL. (C) Trans unsaturated fatty acid. (D) Long Saturated fatty acid. (E) 06 fatty acid.

17. All are true for DNA polymerase   except  : (A) generates dsDNA from ssDNA. (B) synthesizes new strands by adding successive nucleotides in the 5'→3' direction. (C) copies the sequence of nucleotides of one strand in a complementary fashion. (D) requires a primer with a free 5'-OH end, but the 3'-end may be phosphorylated. (E) copies the sequence of nucleotides of one strand to form a new second strand.

18. The higher the ____ content of a DNA, the ____ the melting temperature, and the ____ the ionic strength, the ____ the melting temperature. (A) G:C； higher； lower； lower (B) G:C； lower； higher； lower (C) G:C； higher； higher； lower (D) A:T； higher； higher； lower (E) A:T； lower； lower； higher

19. RNA is ____ stable to alkaline hydrolysis than DNA because RNA's vicinal ____ group makes the 3'-phosphodiester bond susceptible to ________ cleavage. (A) more； 2'-OH； nucleophilic (B) more； 2'-OH； electrophilic (C) less； 2'-OH； nucleophilic (D) less； 3'-OH； nucleophilic (E) more； 3'-OH； electrophilic

21. Alanine is formed from transamination of: (A) asparagine. (B) aspartame. (C) oxaloacetate. (D) citrate. (E) pyruvate.

22. Which statement about ribonucleotide reductase is not true: (A) It is an enzyme that catalyzes the formation of dADP, dGDP, dCDP and dTDP. (B) The reaction catalyzed by ribonucleotide reductase is strictly conserved in all living organisms. (C) The immediate reducing power of ribonucleotide reductase is provided by thioredoxin. (D) Consists of two non-identical subunits. (E) Free radical is involved in the catalysis.

23. Which statement about thymidine kinase (TK) is not true: (A) Thymidine kinase catalyzes the reaction: deoxythymidine + ATP → TMP + ADP. (B) Thymidine kinases have a key function in the synthesis of RNA. (C) Mammals have two isoenzymes, TK1 and TK2. (D) Certain viruses also carry gene for the expression of viral thymidine kinases. (E) None of the above.

24. Many biomolecules are derived from amino acid precursors. Serotonin and dopamine are synthesized from ____ and ____, respectively. (A) histidine； tyrosine. (B) phenylalanine； tryptophan. (C) tryptophan； tyrosine. (D) histidine； tryptophan. (E) tryptophan； histidine.

25. In humans, which of the following amino acids are essential? (A) Gln and Glu. (B) Lys and Cys. (C) Trp and Ser. (D) Arg and Ala. (E) Met and Phe.

26. Choose from the answers 1~6 to describe the following figure, then pick up the correct descriptions. 1. Glc(β1→4)Glc 2. Gal(β1→4)Glc 3. Glc(α1→4)Glc4. Maltose 5. Cellobiose 6. Lactose (A) 2 and 6 are correct. (B) 1 and 4 are correct. (C) 3 and 4 are correct. (D) 1 and 5 are correct. (E) 3 and 5 are correct.