25. The biological role of restriction enzymes is to:
(A) aid recombinant DNA research.
(B) degrade foreign DNA that enters a bacterium.
(C) make bacteria resistant to antibiotics.
(D) restrict the damage to DNA by ultraviolet light.
(E) restrict the size of DNA in certain bacteria.

1. Of the 20 standard amino acids, only  _____ is not optically active. The reason is that its side chain _____. (A) Alanine； is a simple methyl group  . (B) glycine； is unbranched. (C) lysine； contains only nitrogen. (D) glycine； is a hydrogen atom.

2. Thr and/or Leu residues tend to disrupt an α helix when they occur next to each other in a protein because: (A) an amino acid like Thr is highly hydrophobic. (B) covalent interactions may occur between the Thr side chains. (C) steric hindrance occurs between the bulky Thr side chains. (D) the R group of Thr can form a hydrogen bond. (E) electrostatic repulsion occurs between the Thr side chains.

3. An α helix would be destabilized most by: (A) the presence of two Lys residues near the amino terminus of the α helix. (B) interactions between neighboring Asp and Arg residues. (C) interactions between two adjacent hydrophobic Val residues. (D) the presence of an Arg residue near the carboxyl terminus of the α helix. (E) an electric dipole spanning several peptide bonds throughout the α helix.

4. A sequence of amino acids in a certain protein is found to be -Ser-Gly-Pro-Gly-. The sequence is most probably part of a(n): (A) antiparallel β sheet. (B) parallel β sheet. (C) α helix. (D) a sheet. (E) β turn.

5. Which of the following statements about aromatic amino acids is correct? (A) All are strongly hydrophilic. (B) Histidine's ring structure results in its being categorized as aromatic or basic, depending on pH. (C) On a molar basis, tryptophan absorbs more ultraviolet light than tyrosine. (D) The major contribution to the characteristic absorption of light at 280 nm by proteins is the phenylalanine R group. (E) The presence of a ring structure in its R group determines whether or not an amino acid is aromatic.

6. In a mixture of the five proteins listed below, which should elute second in size-exclusion (gel- filtration) chromatography? (A) cytochrome c Mr = 13,000 (B) immunoglobulin G Mr = 145,000 (C) ribonuclease A Mr = 13,700 (D) RNA polymerase Mr = 450,000 (E) serum albumin Mr = 68,500

7. The interactions of ligands with proteins: (A) are relatively nonspecific. (B) are relatively rare in biological systems. (C) are usually irreversible. (D) are usually transient. (E) usually result in the inactivation of the proteins

8. When oxygen binds to a heme-containing protein, the two open coordination bonds of Fe2+ are occupied by: (A) one O atom and one amino acid atom. (B) one O2 molecule and one amino acid atom. (C) one O2 molecule and one heme atom. (D) two O atoms. (E) two O2 molecules.

9. A monoclonal antibody differs from a polyclonal antibody in that monoclonal antibodies: (A) are labeled with chemicals that can be visualized. (B) are produced by cells from the same organism that produced the antigen. (C) are synthesized by a population of identical, or "cloned," cells. (D) are synthesized only in living organisms. (E) have only a single polypeptide chain that can recognize an antigen.

10. Enzymes are potent catalysts because they: (A) are consumed in the reactions they catalyze. (B) are very specific and can prevent the conversion of products back to substrates. (C) drive reactions to completion while other catalysts drive reactions to equilibrium. (D) increase the equilibrium constants for the reactions they catalyze. (E) lower the activation energy for the reactions they catalyze.

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