20. What is the quaternary structure in proteins?
(A) The arrangement of a protein's side chains
(B) The local spatial arrangement of the main-chain atoms in a selected segment of a polypeptide chain
(C) The complete three-dimensional structure of a polypeptide chain
(D) Interactions between the subunits of multi-subunit proteins or large protein assemblies

16. Which of the following is true about protein targeting in eukaryotic cells? (A) Signal sequences are always located at the carboxyl terminus of a newly synthesized protein. (B) Polypeptides with signal sequences are moved into the Golgi apparatus as they are synthesized. (C) Proteins targeted to the nucleus have an internal signal sequence cleaved once the protein is successfully targeted. (D) Proteins targeted to mitochondria and chloroplasts do not use an amino-terminal signal sequence.

17. How does electrophoresis separate proteins? (A) By differences in their primary structure (B) By differences in their tertiary structure (C) By differences in their hydrophobicity (D) By differences in their mass or charge

18. Which of the following is the major contributor to stabilizing the globular form of most soluble proteins? (A) Hydrophobic effect (B) Van der Waals interactions (C) Ionic interactions (D) Hydrogen bonds

19. Which amino acid has two chiral centers? (A) Glycine (B) Tyrosine (C) Valine (D) Isoleucine

21. What is the primary carrier of energy within cells? (A) NADH (B) NADPH (C) FADH2 (D) ATP

22. What is the role of Hsp70 chaperones and chaperonins in protein folding? (A) They catalyze the disulfide-bond formation (B) They facilitate protein folding (C) They degrade misfolded proteins (D) They isomerize Pro peptide bonds

23. What is the primary electron donor in reductive biosynthesis? (A) NADH (B) NADPH (C) FADH2 (D) ATP

24. Which of the following statements is true regarding hemoglobin? (A) Oxygen binding to hemoglobin is not allosteric (B) Oxygen binding to hemoglobin is cooperative and allosteric (C) Hemoglobin exists in a single structural state (D) Hemoglobin contains a single heme prosthetic group

25. Which is NOT a key tool for forming recombinant DNA molecules? (A) Restriction enzyme (B) DNA ligase (C) Plasmid (D) DNA topoisomerases

26. Which immunoglobulin class is the most abundant in humans? (A) IgA (B) IgG (C) IgD (D) IgE