Virus Replication Cycles

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Virus Replication Cycles

• Chapter 3

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One-Step Growth Curves

• New alternative to studying viruses besides
  injecting animals
• Enders, Weller and Robbins developed cell culture
  techniques in the 1940s.
• One-step growth curves used to study a single
  replication cycle of viruses.
• Developed by Delbruck to study E. coli T4
  bacteriophage
• Multiplicity of infection (MOI)
• MOI of 10
• Plaque assays

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Figure 3.1 The steps involved in performing
one-step growth experiments.
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Bacterial Growth vs. Viral Growth
Figure 3.2a Bacterial growth proceeds in a
series of phases lag, log, stationary, and death.
Modified from an illustration by H. Douglas Goff,
Ph.D., University of Guelph
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Bacterial Growth vs. Viral Growth
Adapted from D. E. White and F. J. Fenner.
Medical Virology, Fourth Edition. Academic Press,
1994
Figure 3.2b Viruses require host cells for
growth and reproduction.
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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. naked virus entry

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Figure 3.3a Viral entry steps in ligand mediated
fusion.
Adapted from E. K. Wagner and M. J. Hewlett.
Basic Virology, Second Edition. Blackwell
Publishing, 2003.
Figure 3.3b Viral entry steps in a receptor
mediated endocytotic entry of an enveloped virus.
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Figure 3.4 Steps that naked viruses use to enter
cells.
Adapted from E. K. Wagner and M. J. Hewlett.
Basic Virology, Second Edition. Blackwell
Publishing, 2003.
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• 3. Uncoating (Disassembly and Localization)

Adapted from D. E. White and F. J. Fenner.
Medical Virology, Fourth Edition. Academic Press,
1994
Figure 3.2b Viruses require host cells for
growth and reproduction.
Modified from an illustration by H. Douglas Goff,
Ph.D., University of Guelph
Figure 3.2a Bacterial growth proceeds in a
series of phases lag, log, stationary, and death.
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• 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)

Figure 3-5
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dsDNA Viruses

• Contain dsDNA genome
• Most dsDNA viruses replicate their genomes in the
  nucleus of the cell
• Use hosts DNA and RNA synthesizing machinery

Figure 3-6
Adapted from D. R. Harper. Molecular Virology,
Second Edition. BIOS Scientific Publishers, 1999.
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ssDNA Viruses

• Contain ssDNA genomes

Adapted from D. R. Harper. Molecular Virology,
Second Edition. BIOS Scientific Publishers, 1999.
Figure 3-6b
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ss/dsDNA Viruses (Using an RNA intermediate)

• Virus carries its own reverse transcriptase
• dsDNA enters the nucleus, forms an episome
• Virus does not encode an integrase gene

Figure 3-8
Adapted from D. R. Harper. Molecular Virology,
Second Edition. BIOS Scientific Publishers, 1999.
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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.

Figure 3.9 Differences between positive () and
negative (-) sense ssRNA viral genomes.
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dsRNA viruses

• Contain dsRNA segmented genomes
• Viral polymerase

Figure 3.10 List of dsRNA viruses and their
replication strategy.
Adapted from D. R. Harper. Molecular Virology,
Second Edition. BIOS Scientific Publishers, 1999.
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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

Figure 3.11 List of ssRNA viruses and their
replication strategy.
Adapted from D. R. Harper. Molecular Virology,
Second Edition. BIOS Scientific Publishers, 1999.
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-ssRNA viruses

• Contain -ssRNA segmented or nonsegmented genomes
• Contain a viral RNA-dependent RNA polymerase gene

Figure 3-12b
Figure 3-12a
Adapted from D. R. Harper. Molecular Virology,
Second Edition. BIOS Scientific Publishers, 1999.
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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 hosts chromosome

Adapted from D. R. Harper. Molecular Virology,
Second Edition. BIOS Scientific Publishers, 1999.
Figure 3-13
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• 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

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• 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 envelope proteins
  may undergo a specific proteolytic cleavage, e.g.
  HIV

Figure 3.14 The structural proteins within the
immature HIV virus particle must be cleaved by a
viral protease inside the particle for the virus
to be infectious.
Adapted from S. Vella, et al., AIDS Soc. 4
(1996) 15-18
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• 7. Release
• Newly formed viruses are released to the outside
  environment upon lysis (lytic viruses)
• Latent eukaryotic viruses
• Why dont viruses get stuck on the cellular
  receptors as they are released from the host
  cell?
• Neuraminidase

Figure 3.15 TEM of Measles virus released by
budding.
Courtesy of Shmuel Rozenblatt, Tel Aviv
University, Israel
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3.3 The Error-Prone RNA Polymerase Genetic
Diversity

• RNA viruses mutate or evolve more rapidly than
  DNA viruses.
• RNA Polymerases lack proofreading ability
• Most mutations are lethal
• Some mutations are nonlethal
• Selective advantage

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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

cont.
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Table 3.3, cont. Prevention and Treatment of
Human Viral Diseases Antiviral Drugs
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Table 3.3, cont. Prevention and Treatment of
Human Viral Diseases Antiviral Drugs
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Table 3.3, cont. Prevention and Treatment of
Human Viral Diseases Antiviral Drugs
Drug Virus/Disease Target