Virology Chapter: Viral Entry and Replication

Viral Replication

• Viral replication is a complex process generally divided into phases.
• The early phase of infection involves the virus recognizing a suitable host cell, attaching to it, penetrating its plasma membrane, releasing its genome into the cytoplasm.
• If necessary, the genome is transferred to the nucleus.
• The late phase of infection begins with genome replication and the synthesis of viral macromolecules.
• Viral assembly and release follow these steps.

Learning Objectives

• Students should understand how viruses enter a host cell.
• Viropexis is defined.
• Uncoating of the viral genome is defined.
• The differences between positive-sense and negative-sense RNA viruses.
• Understanding the consequences of plus and minus-sense RNA viruses on the structure of the virus particle.
• Recognition of retroviruses and the differences between early and late viral gene products.
• Comprehension of how viral particles leave infected cells (budding).
• Defining defective interfering particles .
• Viruses as vectors for therapy

Steps in Viral Replication

• Recognition of host cell.
• Attachment of the virus.
• Penetration into the host cell.
• Uncoating of the virus.
• Macromolecular synthesis involving early mRNA and nonstructural proteins synthesis, replication of the genome, late mRNA and structural protein synthesis and posttranslational modification of protein.
• Assembly of the virus.
• Release of enveloped viruses.
• Release of the virus.

How Viruses Enter a Host Cell

• Internalization of a virus into a cell begins with interactions between multiple proteins (VAPs) and cellular receptors on the host cell surface
• The type of cell and structure of the virion (virus particle) influence the internalization mechanism.

How Enveloped Viruses Enter a Host Cell

• Enveloped viruses fuse their membranes with host cellular membranes to release their genome or nucleocapsid directly into the cytoplasm.

How Non-Enveloped Viruses Enter a Host Cell

• Most non-enveloped viruses enter a host cell by receptor-mediated endocytosis or by viropexis.

What is Viropexis

• Viropexis is the process by which certain viruses, including picornaviruses and papovaviruses, enter a cell and are incorporated into it by phagocytosis.
• Exposed hydrophobic structures on the capsid proteins facilitate viral penetration of the cell membrane.

Viral Uncoating

• Uncoating is the physical separation of the viral nucleic acid from the outer components of the virion.
• Host cell enzymes within endosomes or lysosomes are responsible for this separation.
• The infectivity of the parental virus is lost at this point.
• This phase of replication is known as the eclipse period.

Gene Expression and Genome Replication

• Viral genome replication and gene expression occur after uncoating and is largely determined by the genome type.
• RNA viruses typically replicate in the cytoplasm (except for influenza, retroviruses, and HBV, which replicate in the nucleus.)
• DNA viruses typically replicate in the nucleus (except for poxviruses.)

Positive-Sense and Negative-Sense RNA Virus

• Positive-sense RNA viruses can directly be read by ribosomes, resulting in protein synthesis.
• Negative-sense RNA viruses require a viral RNA polymerase, which converts the negative-sense RNA into a positive-sense RNA variant that can be read by ribosomes.

A Positive-Sense RNA Virus

• Positive-sense RNA that comes from a virus is, in itself, an mRNA, immediately ready to be read into ribosomes.
• It moves directly to the ribosome to be translated into protein.

A Negative-Sense RNA Virus

• Negative-sense RNA from the virus itself is unreadable to ribosomes.
• It must first be transformed into positive-sense RNA by a viral RNA polymerase.

Double-Stranded RNA (+/-) Virus

• This type of virus utilizes viral RNA polymerases to produce mRNA.

What is a Retrovirus?

• Retroviruses have a positive-strand RNA genome but cannot replicate it in the cytoplasm.
• A retrovirus carries an RNA-dependent DNA polymerase or reverse transcriptase for this conversion.
• It utilizes this enzyme to convert its RNA into DNA and subsequently integrate it into the host cell's DNA.

Retrovirus (cont'd)

• Retroviruses use reverse transcriptase to translate their genetic information into DNA.
• This allows the DNA to integrate into the host's DNA, enabling the virus to utilize host cell components to produce more viral particles.

How Viral Components Are Synthesized

• Once inside the host cell, the viral genome directs the synthesis of viral mRNA and protein.
• This process generates identical copies of the viral genome.
• The steps of transcription and translation usually take place in stages, involving early and late genes.

Viral Gene Products (Early and Late)

• Early viral proteins are typically DNA-binding proteins and enzymes (like polymerases) that are vital for the initial stages of viral genome replication.
• Fewer copies of early genes are required.
• Late viral proteins are primarily structural proteins essential for packaging and releasing the progeny virions.
• Many copies of these proteins are required.
• Late gene expression generally occurs after genome replication begins.

Assembly and Maturation

• The irreversible combination of viral nucleic acid and protein coat to form new virions.
• Assembly of capsid components by late genes.
• Enveloped viruses require interaction with the modified plasma membrane of the host cell for release.

How New Viral Particles Leave the Infected Cell

• Virus release can occur through lysis (cell death), or via budding (cell survival).
• Non-enveloped viruses typically exit by cell lysis via exocytosis.
• Enveloped viruses often exit by budding, wherein virus-specific envelope proteins are inserted into the host's cellular membrane.

Re-initiation of Viral Replication (Spread of Infection)

• Viruses can spread intracellularly (via fusion of infected cells) or extracellularly into the surrounding medium.
• Multinucleated giant cells (syncytia) are common in some viral infections.

Defining Viral Mutation

• Spontaneous mutations in viral genomes can produce new viable strains with different properties from the wild-type virus.
• These differences include alterations in nucleotide sequences, antigenicity, and pathogenicity.
• Mutations in essential genes can disrupt replication, provide drug resistance, or alter the virus's effect on cells.

Concepts of Viral Mutation and Selection

• Viral mutants are influenced by various factors such as ability to use host cell mechanisms, resistance to host defenses, and cellular properties acting as selecting pressures..
• Viruses that fail in these areas are eliminated from the population.

Defective Interfering Particles (DIPs)

• DIPs are mutant viruses that result from unexpected virus generation.
• A critical portion of the genome may be lost during replication.
• These losses may result from non-homologous recombination.

Viruses as Vectors for Therapy

• Viral genetic modification can be utilized for gene therapy, vaccines, and other treatments.
• Viral vectors, including the attenuated/defective types, are used to deliver genes into host cells.
• In these applications, retroviruses and adenoviruses are often considered.

Consequences of Viral Properties

• Viruses are non-living entities.
• They must be infectious to persist in nature.
• Viruses require host-cell processes to manufacture ( viral messenger RNA, protein and genomes)
• Viruses encode required components not provided by the host cell.
• Viral components must assemble on their own.

Summary

• Concise overview of viral replication, including various steps, including the role of positive- and negative-sense RNA viruses, retroviruses, and viral production, release, and mutations.

Test your knowledge on the key concepts related to viral entry and replication. This quiz covers mechanisms such as genome delivery, viral gene products, and the role of receptors in the viral life cycle. Perfect for students studying virology and related fields.