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.

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.

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.

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.

27. Please choose the correct answer from the following descriptions: 1. Number of hydrogen bonds per water molecule in ice is higher than that in water. 2. 0.1 g   glycogen   in 100 mL water results in lower osmolarity than 0.1 g   glucose   in 100 mL water. 3. Every turn of α-helix is 5.4 Å in length.  4. The strength of α-keratin depends on its Cys content. 5. Hydrogen bond in α-helix formed between residue i and i+3. (A) 1 and 3 are incorrect. (B) 2 and 5 are incorrect. (C) Only 1, 2 and 4 are correct. (D) Only 2 is incorrect. (E) Only 5 is incorrect.

28. A teacher allocates 4 peptides (peptide sequences shown below) to five students: Samuel, John, Eric, Emma, and Ramus (There are two students getting the same peptide). Peptide 1: AKLFPIEARGQA； Peptide 2: CYTTLNKTG； Peptide 3: GTGPPSGNP； Peptide 4: ASQHPMLWR Samuel says: All the residues in my peptide have only one chiral center. John says: My peptide contains many residues which do not favor α-helix structure. Eric says: My peptide does not contain the residue which can be O-glycosylated. Emma says: My peptide can be cleaved into two peptide fragments by trypsin. Ramus says: My peptide does not contain any aromatic ring. Please answer which description is correct: (A) Ramus and Eric get the same peptide. (B) Samuel gets peptide 3 and Emma gets peptide 2. (C) John and Emma get the same peptide. (D) John and Ramus get the same peptide. (E) Eric gets peptide 1 and Samuel gets peptide 2.

29. The following figure describes the torsion angle of the residue i in a protein. What do the following figures tell you? (A) The left one is ψ angle +90 degree. (B) The right one is φ angle +45 degree. (C) The left one is φ angle +90 degree. (D) The right one is φ angle +60 degree. (E) The left one is φ angle -90 degree.

30. A protein mixture contains protein A, B, C. Protein A: homodimer Mr = 180,000, pI 5； Protein B: Mr = 12,000, pI 7； Protein C: M____r = 150,000, pI 9 Joseph says: When I used SDS-PAGE to separate these three proteins. The sequence of the protein bands shown in gel from top to bottom is A-C-B. Steve says: When I used SDS-PAGE to separate these three proteins. The sequence of the protein bands shown in gel from top to bottom is B-A-C Jacky says: When I dissolved the protein mixture in 20 mM phosphate buffer (pH 7) and loaded it to a cation exchange column equilibrated with the same buffer. Only protein C could bind to the column matrix Simon says: When I dissolved the protein mixture in 20 mM phosphate buffer (pH 8) and loaded it to an anion exchange column equilibrated with the same buffer. Only protein C could bind to the column matrix Clarence says: When I dissolved the protein mixture in 150 mM phosphate buffer (pH 7) and used a gel filtration column to separate them, the elution sequence is B→C→A. Rita says: When I dissolved the protein mixture in 150 mM phosphate buffer (pH 7) and used a gel filtration column to separate them, the elution sequence is A→C→B. Lily says: When I dissolved the protein mixture in 150 mM phosphate buffer (pH 7) and used a gel filtration column to separate them, the elution sequence is C→A→B. Who are correct? (A) Joseph, Simon, and Clarence. (D) Simon and Clarence. (B) Rita and Jacky. (E) Joseph, Jacky, and Rita. (C) Steve, Jacky, and Lily.