40. Which one of the following descriptions about the biosynthesis of nucleotides is       correct      ?
(A) The ribosyl phosphate moiety needed for the synthesis of orotidylate, inosinate, and guanylate is provided most directly by ribose 5-phosphate.
(B) Deoxyribonucleotides are derived from the corresponding ribonucleotides by direct reduction at the 3'-carbon atom of the D-ribose.
(C) The immediate precursor of thymidylate (dTMP) is dUMP. The conversion of dUMP to dTMP is a reaction of deamination.
(D) The enzyme xanthine oxidase plays a critical role in the salvage pathways for the formation of nucleotides.
(E) Formation of deoxyribonucleotides is catalyzed by ribonucleotide reductase, in which its substrates are ribonucleoside diphosphates.

37. An elevated insulin level in the blood stimulates synthesis of fatty acids in the liver by: (A) activation of phosphofructokinase-2. (B) activation of phosphofructokinase-1. (C) inactivation of glycogen phosphorylase. (D) activation of acetyl-CoA carboxylase. (E) inhibition of triacylglycerol lipase.

38. Fructose 2,6-bisphosphate (F26BP) is a potent regulator of glycolysis and gluconeogenesis. Which of the following descriptions is       correct      ? (A) The cellular concentration of F26BP is determined by the relative rates of its synthesis by fructose 2,6-biphosphatase (FBPase-2) and breakdown by phosphofructokinase-2 (PFK-2). (B) F26BP is an allosteric activator for the enzyme fructose 1,6-biphosphatase (FBPase-1). It activates FBPase-1 and stimulates glycosis in liver. (C) Insulin increases the cellular concentration of F26BP by activating a cAMP-dependent kinase that phosphorylates PFK-2. (D) Phosphorylation of the bifunctional enzyme PFK-2 and FBPase-2 inhibits PFK-2 activity and stimulates FBPase-2 activity, leading to breakdown of F26BP. (E) An elevated level of glucagon or epinephrine increases the cellular concentration of F26BP.

39. One distinction between peptide and steroid hormones is that steroid hormones: (A) bind to their receptors with low affinity, whereas peptide hormones bind with high affinity. (B) bind to nuclear receptors, whereas peptide hormones bind to cell surface receptors. (C) are less stable than peptide hormones. (D) are generally water-soluble, whereas peptide hormones are water-insoluble. (E) act through specific receptors, whereas peptide hormones act through nonspecific receptors.

41. Which of the following statements is       correct      ? (A) When a solution of DNA is heated slowly until the tm is reached, the DNA molecules are completely denatured at this temperature. (B) In double-stranded DNA, only a right-handed helix is possib (C) The pentoses found in nucleic acids are always in the β-furanose forms. (D) The nucleic acid bases absorb ultraviolet light maximally at 280 nm. (E) In double-stranded DNA, the sequence of bases has no effect on the overall structure.

42. Introns: (A) are frequently present in prokaryotic genes but are rare in eukaryotic genes. (B) are spliced out before transcription. (C) are translated but not transcribed. (D) can occur many times within a single gene. (E) encode unusual amino acids in proteins.

43. Which of the following evidence is indicative of the presence of a gene in an unknown DNA sequence? (A) alignment to a partial cDNA sequence. (B) sequence similarity to genes of other organisms. (C) ORF consistent with the rules for exon and intron sequences. (D) all of the above. (E) none of the above.

44. How does binding of the lac repressor to the lac operator block transcription initiation? (A) lac repressor binding blocks RNA polymerase from interacting with DNA at the start site. (B) lac repressor binding induces a DNase that cleaves the DNA at the transcription start site. (C) lac repressor binding causes a conformational change in RNA polymerase. (D) lac repressor binding induces a protease that degrades the sigma subunit of RNA polymerase. (E) all of the above.

45. An enhancer (A) binds to RNA polymerase and stimulates transcription. (B) is a DNA element that stimulates transcription of eukaryotic promoters. (C) acts as a binding site for RNA polymerase. (D) interacts with repressor proteins to enhance transcriptional repression. (E) all of the above.

46. Which of the following is a protein that is involved in translation? (A) topoisomerase. (B) ribosomal RNA. (C) RNA polymerase. (D) aminoacyl-tRNA synthetase. (E) none of the above.

47. An enzyme that breaks DNA, dispels the tension, and reseals the strand ahead of a DNA replication growing fork is called a(n) (A) topoisomerase. (B) DNA polymerase. (C) phosphodiesterase. (D) aminoacyl-tRNA synthetase. (E) all of the above.

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