Chapter Outlines

Chapter 3      Virus Replication Cycles

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3.1 One-Step Growth Curves
  • New alternative to studying viruses besides injecting animals:
    • Enders, Weller and Robbins developed cell culture techniques in the 1940’s.
  • One-step growth curves used to study a single replication cycle of viruses.
      • Developed by Delbruck to study E. coli T4 bacteriophage
    • MOI
    • Plaque assays
Bacterial Growth vs. Viral Growth
3.2 Key Steps of the Viral Replication Cycle
1. Attachment (adsorption)
  • Host range
  • Cell surface receptors
Proteins, glyoproteins, carbohydrates, lipids
  • Co-receptors
2. Penetration (entry)
  • Clathrin-coated pits
  • Endosomes
  • pH dependent or pH independent
  • Enveloped virus entry vs. Nake virus entry
3. Uncoating (Disassembly and Localization)
4. Types of Viral Genomes and Their Replication
  • Two events critical to viral infection:
    • The production of virus structural proteins and enzymes
    • Replication of the viral genome (dsDNA, ssDNA, dsRNA, ssRNA)
dsDNA Viruses
  • Contain dsDNA genome
  • Most dsDNA viruses replicate their genomes in the nucleus of the cell
    • Use host’s DNA and RNA synthesizing machinery
ssDNA Viruses
  • Contain ssDNA genomes
ss/dsDNA Viruses (Using an RNA intermediate)
  • Virus carries it’s own reverse transcriptase
  • dsDNA enters the nucleus, forms an episome
  • Virus does not encode an integrase gene
RNA Viruses
  • Genomes may be ss or ds, (+) or (-) sense
  • The type of genome determines if the first step after uncoating will be translation, transcription, or RNA replication.
  • RNA viruses carry an RNA-dependent RNA polymerase that will synthesize viral genomes into the host cell with them.
dsRNA viruses
  • Contain dsRNA segmented genomes
  • Viral polymerase
+ssRNA Viruses
  • Contain +ssRNA nonsegmented genomes
  • The RNA in the virus particle functions as mRNA
  • Viral mRNA is recognized by cellular translational machinery
  • Contain a viral RNA-dependent RNA polymerase in order to replicate viral genomes
-ssRNA viruses
  • Contain -ssRNA segmented or nonsegmented genomes
  • Contain a viral RNA-dependent RNA polymerase gene
Viruses with ssRNA Genomes That Use a dsDNA Intermediate to Replicate
  • Unique biology
  • Viral genome is reverse transcribed and integrated as a cDNA into the host’s chromosome
5. Assembly
  • All of the components of the virus assembled into a particle
  • Occurs when an appropriate concentration of virus proteins and genomic nucleic acids are reached and localized at specific sites within the infected cell
  • Some particles self-assemble
6. Maturation
  • Stage in the life cycle of the virus when it becomes infectious
  • Viral or cellular proteases often involved
    • One or more of the capsid or envelop proteins may undergo a specific proteolytic cleavage
    7. Release
    • Newly formed viruses are released to the outside environment upon lysis (lytic viruses)
    • Latent eukaryotic viruses
    • Why don’t viruses get stuck on the cellular receptors as they are released from the host cell.
    3.3 The Error-Prone RNA Polymerase
    Genetic Diversity
    • RNA viruses mutate or evolve more rapidly than DNA viruses.
      • RNA Polymerases lack proofreading ability
    3.4 Targets for Antiviral Therapies
    • Any of the 7 stages of the virus life cycle can be targeted for antiviral intervention:
      • 1. Attachment
      • 2. Penetration
      • 3. Uncoating
      • 4. Replication
      • 5. Assembly
      • 6. Maturation
      • 7. Release

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