Viral Replication Overview

Viral Replication

• Viruses replicate in three phases: early, late, and immediate early.
• Viral replication involves several steps, starting with attachment to the host cell; followed by penetration, uncoating, and the synthesis of viral components like mRNA and proteins.
• Replication also involves transcription into new viral components and replication of the genome.
• Assembly of the virus and release from the cell completes the process.

Learning Objectives

• Students should be able to describe the entry of a virus into a host cell.
• Viropexis is the mechanism of a virus entering a cell by phagocytosis.
• Uncoating of the viral genome is the process of releasing the viral nucleic acid from the outer structural components of the virus.
• Determine the difference between positive-sense and negative-sense RNA viruses.
• Evaluate the impact of different types of viral RNA structures on viral particle composition.
• Describe how retroviruses function.
• Understand the process of viral component synthesis.
• Define the concept of mutation and selection in viruses.
• Explain how viruses act as therapeutic vectors.

Introduction

• Viral replication is generally divided into phases.
• The early phase involves recognition, attachment, penetration into a cell, release of the genetic material into the cytoplasm, and optionally, movement of viral materials to the nucleus.
• The late phase of replication starts with the replication of the viral genome and viral macromolecular synthesis. This phase culminates with viral assembly and release.

Steps in Viral Replication

• Recognition of the target cell
• Attachment
• Penetration
• Uncoating
• Macromolecular synthesis
• Replication of viral genome
• Post-translational protein modification
• Viral assembly
• Budding of enveloped viruses
• Release of the virus

How a Virus Enters its Host Cell

• Internalization of viruses into a host cell is reliant on the interaction between virus-associated entry proteins and cellular receptors.
• The internalization mechanism depends on the viral structure and the type of host cell.
• Enveloped viruses fuse their membranes with host cell membranes.
• Non-enveloped viruses use receptor-mediated endocytosis or viropexis.

What is Viropexis?

• Viropexis is the process in which specific classes of viruses (e.g., picornaviruses and papovaviruses) are incorporated into a cell via phagocytosis.
• This process is enabled by exposed hydrophobic structures present in capsid proteins, aiding viral penetration into the cell membrane.

Viral Replication Cycle

• Adsorption (attachment)
• Penetration
• Uncoating
• Production of virion components
• Transcription
• Genome Replication
• Assembling of naked capsid viruses and nucleocapsids
• Release of viral particles

Attachment or Adsorption

• Viruses need to acknowledge and bind to specific receptors on the host cell surface.
• Receptor molecules vary between virus types, typically presenting as glycoproteins.
• Receptors can also be proteins or oligosaccharides.
• Viruses show specificity for cells and tissues based on the receptor they target. (e.g., HIV targets CD4 proteins, rabies targets acetylcholine receptors, and polio targets CNS cells.)

Penetration

• Following binding, the virus particle is internalized into the host cell.
• The internalization mechanism depends on the virus's structure.
• Non-enveloped viruses are internalized via endocytosis.
• Enveloped viruses utilize membrane fusion to enter the cell.

Uncoating of the Viral Genome

• The viral genome needs to be uncoated after entering the host cell to reach its replication site.
• This process involves the removal of the capsid or envelope by host enzymes (found within endosomes or lysosomes).
• Viral infectivity in lost after the uncoating process is finished.
• Uncoating initiates a period known as "eclipse period" during which the virus is not infectious. This phase is followed by a substantial period of rapid viral particle replication.

Gene Expression and Genome Replication

• This phase occurs after uncoating.
• Most RNA viruses complete their replication cycles within the cytoplasm, with exceptions such as retroviruses, influenza viruses, and hepatitis D viruses.
• Viral DNA replication takes place within the nucleus (exceptions include poxviruses and herpesviruses).

Positive-Sense and Negative-Sense RNA Virus

• Positive-sense RNA viruses have a genome directly translatable into viral proteins by cellular ribosomes.
• Negative-sense RNA viruses need an RNA-dependent RNA polymerase to produce complementary mRNA.

A Positive-Sense RNA Virus

• The positive-sense RNA in a virus serves as mRNA, directly entering ribosomes for protein synthesis.

A Negative-Sense RNA Virus

• Negative-sense RNA viruses require a viral enzyme to produce mRNA for translation.

Double-Stranded RNA (dsRNA)

• These viruses use viral RNA polymerases to produce mRNA.

What is a Retrovirus?

• Retroviruses are positive-strand RNA viruses that utilize reverse transcriptase.
• This process converts their RNA genome into DNA, which is then integrated into the host cell DNA.
• Retroviruses carry two copies of their RNA, transfer RNAs, and reverse transcriptase.

Retrovirus (Continued)

• Retroviruses use reverse transcriptase to translate their genetic information into DNA, integrate into the host cell DNA.
• This allows them to take advantage of the host cell's machinery for the production of new viral particles.

How Viral Components are Synthesized

• Viral genomes direct the host cell's processes for synthesis of viral mRNA and proteins.
• The genome creates identical copies of itself.
• mRNAs are converted into proteins through translation.

Transcription and Translation Steps in Two Stages (Early and Late)

• Early proteins, such as polymerases and regulatory proteins, are produced in one stage.
• Late proteins, including structural proteins and those involved in viral release, are created in the subsequent stage.

Viral Macromolecular Synthesis Steps

• Double-stranded DNA copies host processes to form mRNA.
• Single-stranded DNA converts to double-stranded DNA.
• Positive-sense RNA directly functions as mRNA.
• Negative-sense RNA requires an RNA-dependent polymerase, producing mRNA.
• Double-stranded RNA viruses also utilize RNA polymerases to make mRNA.

Differences Between Early and Late Viral Gene Products

• Early products often include DNA-binding proteins and enzymes, including virus-encoded polymerases.
• Early proteins usually are needed earlier in infection to enable replication.
• Late products are structural proteins, necessary for complete virion assembly.
• Late proteins needed for infectious particle packaging are often made after genome replication.

Replication of a Complex Enveloped DNA Virus

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

Assembly and Maturation

• This final step involves the combination of viral nucleic acid and proteins to form new infectious virions.
• For enveloped viruses, interaction with modified plasma membranes is necessary.

How New Viral Particles Leave the Infected Cell

• There are two primary methods:
• Lysis, where host cell death occurs after generating viral particles.
• Budding, where viral particles are released from the infected cell without causing immediate cell death, allowing continual production and release.
• Non-enveloped viruses are released by exocytosis.
• Enveloped viruses are released by budding. This method involves the virus using host-cell plasma membrane to form its new envelope.

Re-initiation of Replication (Spread of Infection)

• The virus is released from the host cell and/or infectious agents.
• Viruses can spread through cell-to-cell fusion, resulting in larger or multinucleated infected cells.
• These spread through cell division to neighboring cells.

Defining the Concept of Viral Mutation

• Viral genomes change spontaneously and frequently, producing new viral strains.
• The newly formed viral strains could exhibit different characteristics compared to their precursors.
• Mutations can also affect the virus's interactions with host cells causing inactivating mutations.
• Mutations can drive drug resistance or changes to pathogenicity.

Concepts of Mutation and Selection

• Viral mutations are influenced by the virus's ability to use the host cell process, environmental conditions, cellular characteristics, and tissue-specific proteins required by the virus.
• Viruses that cannot handle pressures of the host defenses/responses are often eliminated through selection.

What are Defective Interfering Particles (DIPs)?

• DIPs are mutated viruses that are defective or incomplete.
• They arise due to errors in replication or recombination within the host cell.
• DIPs result in the loss of a critical part of the genome.

Viruses as Vectors for Therapy

• Manipulating the viral genetic composition offers opportunities for gene replacement therapy.
• Viruses can be modified for use as vaccines.
• Some viral genes are modified to remove damaging aspects of the virus to be used therapeutically.

Consequences of Viral Properties

• Viruses are non-living, and require host cell processes to reproduce.
• Viruses have a self-assembly life cycle from the moment of infectious entry into a host cell.

Steps in Viral Replication (Summary)

• Recognition of the target cell
• Attachment
• Penetration
• Uncoating
• Macromolecular synthesis
• Replication of genome
• Post-translational protein modification, packaging
• Assembly
• Budding of enveloped viruses
• Release of virus.

References

• Provide references for the information, including publication details (authors, year, book title, page numbers).

This quiz covers the complex process of viral replication, highlighting the three key phases: early, late, and immediate early. Students will learn about the mechanisms of entry into host cells, uncoating, and the synthesis of viral components, as well as the distinctions between various types of RNA viruses.