Virology Quiz: Viral Replication and Entry Mechanisms

Viral Replication

• Viral replication is a process of separating into phases
• Early phase infection:
  • Recognition of the appropriate target cell
  • Attachment to the cell
  • Penetrating the plasma membrane
  • Release (uncoating) of the genome into the cytoplasm
  • Delivery of the genome to the nucleus (if needed)
• Late phase infection:
  • Begins with genome replication and viral macromolecular synthesis
  • Proceeds through viral assembly and release
• The steps in viral replication include:
  • Recognition of the target cell
  • Attachment
  • Penetration
  • Uncoating
  • Macromolecular synthesis (early and late mRNA, replication of the genome, and protein synthesis and modification)
  • Assembly of the virus
  • Budding (for enveloped viruses)
  • Release of the virus

How a Virus Enters its Host Cell

• Internalization of the virus into a cell requires
  • Interactions between VAPs and cellular receptors
  • Structure of the virion
  • Cell type
• Envelope viruses: Fuse their membranes with cellular membranes, releasing the genome/nucleocapsid directly into the cytoplasm
• Non-envelope viruses: Enter cells through receptor-mediated endocytosis or viropexis

Viropexis

• Defined as the process by which some viruses (e.g., picornaviruses and Papovaviruses) enter a cell and are internalized by phagocytosis
• Facilitated by exposed hydrophobic structures on the capsid proteins

Uncoating

• A physical separation of the viral nucleic acid from the outer structural components of the virion
• Facilitated by host cell enzymes in endosomes or lysosomes
• The infectivity of the parental virus is lost

Gene Expression and Genome Replication

• Happens after viral genome uncoating
• Most RNA viruses complete replication in the cytoplasm, except for retroviruses, influenza viruses, and hepatitis D virus
• Viral DNA replicates in the nucleus, except for poxviruses and herpes viruses

Positive-sense and Negative-sense RNA Viruses

• Positive-sense RNA virus: The viral RNA acts as mRNA, directly translated into protein by the ribosome
• Negative-sense RNA virus: Requires an RNA polymerase to convert the viral RNA into mRNA for protein translation

Double-stranded RNA Viruses

• Use viral RNA polymerases to synthesize mRNA

Retroviruses

• Positive-strand RNA genome
• No means for replication in the cytoplasm
• Carry two copies of the genome, tRNA molecules, and RNA-dependent DNA polymerase (reverse transcriptase)
• Reverse transcriptase translates the RNA into DNA, allowing the viral DNA to be integrated into the host cell DNA

Viral Components Synthesis

• The viral genome directs the synthesis of viral mRNA and protein
• The viral genome creates identical copies of itself
• Transcription translates the viral genetic information into mRNA
• Translation converts mRNA into protein

Transcription and Translation Stages

• Early proteins: Polymerases, regulatory proteins, viral antigens
• Late proteins: Structural capsid proteins, enzymes for viral release

Viral Macromolecular Synthesis Steps

• Double-stranded DNA (dsDNA) uses host machinery to produce mRNA (except for poxviruses)
• Single-stranded DNA (ssDNA) converts to dsDNA and replicates like dsDNA
• Positive-sense RNA (mRNA-like) is translated into protein by ribosomes
• Negative-sense RNA requires RNA polymerase to make mRNA

Differences between Early and Late Viral Gene Products

• Early gene products are often DNA-binding proteins and enzymes (including viral polymerases), typically present in smaller quantities
• Late gene products are structural proteins and other proteins essential for viral packaging and are not required before genome replication

Replication of a Complex Enveloped DNA Virus

• Transcription and translation occur in three phases: immediate early, early, and late

Assembly and Maturation

• Irreversible combination of viral nucleic acid and protein coat to form new-infectious virions
• Capsid synthesis is directed by late genes
• Enveloped virus assembly needs plasma membrane interaction

Release of Viral Particles

• Lysis: Cytolytic viruses cause cell death to release viruses
• Budding: Cytopathic viruses do not kill infected cells; viruses bud off from the cell using plasma membrane
• Non-enveloped viruses are released via exocytosis

Re-initiation of Replication (Spread)

• Viral release to the extracellular medium
• Through cell-to-cell fusion
• Into multinucleated giant cells (syncytia) cells
• Becoming huge virus factories
• Vertically into daughter cells

Defining the Concept of Viral Mutation

• Spontaneous and frequent in viral genomes
• Resulting in new strains of viruses with differing properties (compared to the parental form)
• Mutations in essential genes can:
  • Inactivate the virus (lethal mutations)
  • Cause antiviral drug resistance
  • Alter viral antigenicity or pathogenicity

Mutation and Selection

• Viral mutants are selected based on their ability to use host cell machinery and withstand conditions
• Cellular properties act as pressure selections
• Includes cell growth rate and tissue-specific protein requirements (e.g., enzymes, glycoproteins, transcription factors)
• Viruses that cannot adapt or escape the host's defense mechanisms are eliminated

Defective Interfering Particles (DIPs)

• Also known as defective interfering viruses or mutants
• Result from unexpected virus generation
• Loss of a critical portion of the genome either from defective replication or non-homologous recombination

Viruses as Vectors for Therapy

• Modification of viral genetic composition is used for:
  • Gene replacement therapies
  • Vaccines to stimulate immunity
  • Replacing harmful genes with desired ones
  • Using viral vectors (attenuated/defective) in clinical trials (e.g., retroviruses or adenoviruses).

Consequences of Viral Properties

• Viruses lack life (non-living)
• Viruses must be infectious to survive
• Viruses utilize host cellular processes to generate components (mRNA, proteins, identical genome copies)
• Viruses must encode required functions not present in the cell
• Viral components self-assemble

Steps in viral replication

• Recognition of the target cell
• Attachment
• Penetration
• Uncoating
• Macromolecular synthesis (early mRNA, replication of the genome, late mRNA, posttranslational modifications of protein)
• Assembly of the virus
• Budding of enveloped viruses (if present)
• Viral release