42. The primary source of one-carbon units for tetrahydrofolate in the cells is the carbon removed in the conversion of to homocystein.
(A) methionine to homocystein.
(B) serine to pyruvate.
(C) N-methyltetrahydrofolate to tetrahydrofolate.
(D) serine to glycine.
(E) S-adenosylmethionine to S-adenosylhomocystein.

38. Assuming that you are attending a triathlon, which one will     not     occur? (A) Ketone bodies are accumulating in the brain. (B) Proton pump is increasing concentration in the stomach. (C) Bicarbonate is increasing in the blood. (D) Aquaporin is lowering its water reabsorption in the renal tubes. (E) Malonyl-CoA is inhibiting ẞ oxidation.

39. Which of the following statements is     false    ? (A) Animals can produce tyrosine directly from phenylalanine. (B) 1-Dopa is an intermediate in the conversion of tyrosine to epinephrine. (C) Bile pigments are formed in the degradation of heme. (D) Histidine undergoes amination to histamine. (E) Asparagine can be synthesized by amidation of aspartate, thus it is not an essential amino acid in human.

40. Which of the following descriptions is      false    ? (A) When a solution of double-stranded DNA is heated at neutral pH, the absorption of ultraviolet (260 nm) light may increase. (B) Double-stranded regions of RNA can form between two self-complementary regions of the same single strand of RNA. (C) Lowering the ionic strength of a solution of double-stranded DNA will decrease the melting temperature of the DNA: (D) The Hoogsteen (non Watson-Crick) interaction, which can result in triple-helical DNA structures, are primarily hydrophobic interactions involving the bases. (E) In nucleotides and nucleic acids, syn and anti conformations relate to rotation around the sugar-base bond.

41. Which of the following statements is     correct     of the biosynthetic pathways for purine and pyrimidine nucleotides? (1) The amino acid aspartate is directly involved in the purine biosynthetic pathway. (2) 5-Phosphoribosyl-1-pyrophosphate (PRPP) is a synthetic precursor for UMP. (3) Orotic acid is an essential precursor for purine nucleotides.  (4) Inosinate is the precursor for the synthesis of pyrimidine nucleotides. (5) The carbamoyl phosphate required in pyrimidine biosynthesis is made in the cytosol by carbamoyl phosphate synthetase II. (A) 1 and 4. (B) 1, 2, and 3. (C) 4 and 5. (D) 1, 2, and 5. (E) 2 only.

43. Which of the following statements regarding plasmid cloning vectors is     correct    ? (1) Foreign DNA fragments up to 30,000 base pairs cannot be cloned in a typical plasmid vector. (2) Plasmid vectors must carry promoters for inserted gene fragments. (3) The copy number of plasmids may vary from a few to several hundred. (4) In the laboratory, microinjection is commonly used to introduce recombinant plasmids into bacterial cells. (5) The replication origin (ori sequence) in plasmid cloning vectors is a sequence required for packaging recombinant plasmids into bacteriophage. (A) 1 and 2. (B) 3 and 5. (C) 1, 2, and 4. (D) 1 and 3. (E) 3 only.

44. Which of the following sequences is required in an expression vector (for use with E. coli) that is     not     essential in a general cloning plasmid? (1) replication origin (2) a bacterial promoter and its associated operator (3) a ribosomal binding site (4) a transcription termination sequence (5) antibiotic resistance gene (A) 1 and 2. (B) 2, 4, and 5. (C) 2, 3, and 4. (D) 2 and 4. (E) 2 only.

45. Many important neurotransmitters are primary or secondary amines, derived from amino acids through decarboxylation. For example, glutamate decarboxylation gives rise to y-aminobutyrate (GABA). These amino acid decarboxylation reactions require the coenzyme: (A) pyridoxal phosphate (PLP). (B) biotin. (C) . (D) tetrahydrofolate. (E) nicotinamide adenine dinucleotide (NAD).

46. The enzymes having the prosthetic group of pyridoxal phosphate (PLP) participate in a variety of reactions at the a carbons of amino acids. These reactions include: (1) racemization (2) decarboxylation (3) transmination (4) methylation (5) acetylation (A) 3 and 4. (B) 1, 2, and 3. (C) 3 and 5. (D) 1 and 3. (E) 3 only.

47. Serine can be converted directly into pyruvate. The reaction, catalyzed by serine dehydratase, requires the coenzyme: (A) pyridoxal phosphate (PLP). (B) biotin. (C) . (D) tetrahydrofolate. (E) nicotinamide adenine dinucleotide (NAD).

48. Which of the following descriptions about human metabolism is    correct   ? (1) The Cori cycle is the synthesis of alanine from pyruvate in skeletal muscle and the synthesis of pyruvate from alanine in liver. (2) Glucokinase in the liver acts in the conversion of liver glycogen to glucose 1-phosphate. (3) The largest energy store in a well-nourished human is liver glycogen. (4) Among its numerous metabolic effects, the protein leptin makes muscle and liver cells more sensitive to insulin. (5) The hormone insulin decreases appetite. (A) 1 and 4. (B) 2 and 5. (C) 1 and 3. (D) 4 and 5. (E) 4 only.