A pharmaceutical researcher is trying to develop a vaccine against the hepatitis C virus. She infects a chimpanzee with hepatitis C virus of known genotype and subtype. Several weeks later, a liver sample is obtained, and viral RNAs are extracted from the hepatocytes. A genetic study of the viral genomes reveals that the extracted RNA sequences vary significantly from that of the original infecting virus. This genetic instability is most likely due to the lack of which of the following features during the viral replication process?

Question

A pharmaceutical researcher is trying to develop a vaccine against the hepatitis C virus.  She infects a chimpanzee with hepatitis C virus of known genotype and subtype.  Several weeks later, a liver sample is obtained, and viral RNAs are extracted from the hepatocytes.  A genetic study of the viral genomes reveals that the extracted RNA sequences vary significantly from that of the original infecting virus.  This genetic instability is most likely due to the lack of which of the following features during the viral replication process?

Quizplus · Accepted Answer

Hepatitis C virus (HCV) has >6 genotypes and multiple subgenotypes. These variant strains differ primarily at hypervariable genomic regions, such as those found in the sequences encoding its 2 envelope glycoproteins. Moreover, there is no proofreading 3' → 5' exonuclease activity built into the virion-encoded RNA polymerase. As a result, the RNA polymerase makes many errors during replication, and several dozen subspecies of HCV are typically present in the blood of an infected individual at any one time. (Choice B) 5' → 3' exonuclease activity allows DNA polymerase I to engage in nick translation, which is important for both DNA repair and removal of RNA primers during replication. This feature is not the cause of instability in HCV. (Choice C) DNA glycosylase is important in base excision repair as it removes the nitrogen base from the sugar-phosphate backbone. An endonuclease then completes the remainder of the repair. This enzyme is not the cause of instability in HCV. (Choice D) DNA ligase is used to join 2 fragments of DNA and is therefore necessary for both DNA repair and replication. The HCV genome is a single-stranded, positive-sense RNA molecule that does not require ligase activity during replication. (Choice E) Nucleotide specificity refers to the importance of inserting the correct nucleotide (eg, adenine, guanine, cytosine, thymine, uracil) into a strand of DNA or RNA. It is not the cause of instability in HCV. (Choice F) DNA primase is a form of RNA polymerase. In bacteria, it binds with DNA helicase and synthesizes a short RNA primer to which nucleotides can be added by DNA polymerase. This enzyme is not the cause of instability in HCV. Educational objective: The hepatitis C virus is genetically unstable because it lacks proofreading 3' → 5' exonuclease activity in its RNA polymerase. Its envelope glycoprotein sequences also contain a hypervariable region prone to frequent genetic mutation.

A Pharmaceutical Researcher Is Trying to Develop a Vaccine Against