Microbiology Fundamentals: Viral Genomes and Replication

Overview

In this section, we explore viral genomes, focusing on the differences between RNA and DNA viruses, the concepts of positive and negative sense RNA, and replication processes. We also examine how viral envelopes form and the infectivity of purified viral genomes.

Types of Viral Genomes

• RNA Viruses
  • Typically single-stranded.
  • Exception: Reoviruses are double-stranded RNA.
• DNA Viruses
  • Typically double-stranded.
  • Exception: Parvovirus is single-stranded DNA.

Positive Sense vs. Negative Sense RNA

• Positive Sense RNA
  • Functions like eukaryotic mRNA.
  • Can be directly translated by host ribosomes in the 5’ to 3’ direction.
• Negative Sense RNA
  • Cannot be directly translated; must first be converted to positive sense.
  • Requires its own RNA-dependent RNA polymerase (RdRp) to generate a positive strand since eukaryotic cells lack this enzyme.

Viral Genome Infectivity

• Infectious Purified Viral Genomes
  • Positive stranded RNA viruses.
  • DNA viruses, except Poxviruses and Hepatitis B (HBV).
  • Infectivity is mainly a research setting phenomenon, requiring direct injection into host cells.
• Non-infectious after Purification
  • Negative stranded RNA viruses.
  • Require RdRp, which is not present after purification.
  • Poxviruses and HBV require specific polymerases.

Special Considerations for Poxviruses and HBV

• Poxviruses
  • Replicate in cytoplasm, not nucleus.
  • Require a DNA-dependent RNA polymerase because the host polymerase stays in the nucleus.
• Hepatitis B Virus (HBV)
  • Requires its own DNA polymerase during its lifecycle.
  • Utilizes reverse transcriptase (RNA-dependent DNA polymerase).

Viral Replication and Translation

• Translation
  • Results in new proteins essential for virion assembly like capsids, polymerases, etc.
• Replication
  • RNA viruses: Occurs in the cytoplasm.
  • DNA viruses: Occurs in the nucleus (except for Poxviruses).

Polymerases

• RNA-dependent RNA polymerase (RdRp)
  • Converts negative sense RNA to positive.
  • Found in negative sense RNA viruses.
• RNA-dependent DNA Polymerase (Reverse Transcriptase)
  • Converts RNA into DNA.
  • Found in retroviruses (e.g., HIV, HTLV) and HBV.

Viral Envelopes

• Formation
  • Typically acquired from the host cell's lipid bilayer during virion exit.
  • Exception: Herpes viruses acquire envelope from the nuclear membrane.

Quiz Application

• Understanding viral lifecycles and envelope formation can help identify viral characteristics in experimental questions.
  • Example: Identifying lipid membrane fusion as a penetration method for enveloped viruses.

This comprehensive overview of viral genomes and replication is crucial for understanding the fundamental processes and exceptions in viral biology.