40. DNA in a closed-circular, double-stranded molecule with no net bending of the DNA axis on itself is:
(A) a left-handed helix.
(B) a mixed right- and left-handed helix.
(C) relaxed.
(D) supercoiled.
(E) underwound.

41. The 5'→3' exonuclease activity of E. coli DNA polymerase I is involved in: (A) formation of a nick at the DNA replication origin. (B) formation of Okazaki fragments. (C) proofreading of the replication process. (D) removal of RNA primers by nick translation. (E) sealing of nicks by ligase action.

42. In base-excision repair, the first enzyme to act is: (A) AP endonuclease. (B) Dam methylase. (C) DNA glycosylase. (D) DNA ligase. (E) DNA polymerase.

43. Which one of the following statements about the reverse transcriptases of retroviruses and the RNA replicases of other single-stranded RNA viruses, such as R17 and influenza virus, is correct? (A) Both enzymes can synthesize either RNA or DNA from an RNA template strand. (B) Both enzymes can utilize DNA in addition to RNA as a template strand. (C) Both enzymes carry the specificity for the RNA of their own virus. (D) Both enzymes have error rates similar to those of cellular RNA polymerases. (E) Both enzymes require host-encoded subunits for their replication function.

44. Differential RNA processing may result in: (A) a shift in the ratio of mRNA produced from two adjacent genes. (B) attachment of the tail to the 5' end of an mRNA. (C) inversion of certain exons in the final mRNA. (D) the production of the same protein from two different genes. (E) the production of two distinct proteins from a single gene.

45. Which of the following is not known to be involved in initiation by eukaryotic RNA polymerase II? (A) DNA helicase activity. (B) DNA polymerase activity. (C) Formation of an open complex. (D) Protein binding to specific DNA sequences. (E) Protein phosphorylation.

46. The sigma factor of E. coli RNA polymerase: (A) associates with the promoter before binding core enzyme. (B) combines with the core enzyme to confer specific binding to a promoter. (C) is inseparable from the core enzyme. (D) is required for termination of an RNA chain. (E) will catalyze synthesis of RNA from both DNA template strands in the absence of the core enzyme.

47. Which of the following statements correctly describes promoters in E. coli? (A) A promoter may be present on either side of a gene or in the middle of it. (B) All promoters have the same sequence that is recognized by RNA polymerase holoenzyme. (C) Every promoter has a different sequence, with little or no resemblance to other promoters. (D) Many promoters are similar and resemble a consensus sequence, which has the highest affinity for RNA polymerase holoenzyme. (E) Promoters are not essential for gene transcription, but can increase its rate by two- to three-fold.

48. Transcription of the lactose operon in E. coli is stimulated by: (A) a mutation in the repressor gene that strengthens the affinity of the repressor for the operator. (B) a mutation in the repressor gene that weakens the affinity of the repressor for the operator. (C) a mutation in the repressor gene that weakens the affinity of the repressor for the inducer. (D) binding of the repressor to the operator. (E) the presence of glucose in the growth medium.

49. The function of the eukaryotic DNA replication factor PCNA (proliferating cell nuclear antigen) is similar to that ofthe ẞ-subunit of bacterial (A) DNA polymerase III in that it: facilitates replication of telomeres. (B) forms a circular sliding clamp to increase the processivity of replication. (C) has a 3'→ 5' proofreading activity. (D) increases the speed but not the processivity of the replication complex. (E) participates in DNA repair.

50. Protein structural motifs often have general functions in common. Which one of the following motifs is known to be involved in protein dimer formation, but not in direct protein-DNA interactions? (A) B-barrel. (D) leucine zipper. (B) helix-turn-helix. (E) zinc finger. (C) homeodomain.

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