Virology Chapter: Viral Entry and Replication

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Questions and Answers

What is the term used to describe the mechanism by which a virus enters a host cell involving direct delivery of its genome into the cytoplasm?

Endocytosis

Viropexis (correct)

Mitosis

Transcription

Which phase of viral replication includes genome replication and viral macromolecular synthesis?

Late phase (correct)

Release phase

Early phase

Maturation phase

How do envelope viruses typically enter a host cell?

By receptor-mediated endocytosis

By fusing their membrane with cellular membranes (correct)

By directly penetrating the plasma membrane

By active transport mechanisms

What is the role of viral-associated proteins (VAPs) in viral entry into a host cell?

They mediate the interaction with cellular receptors. Signup and view all the answers

Which of the following correctly distinguishes between positive-sense and negative-sense RNA viruses?

Positive-sense RNA can be directly translated into proteins; negative-sense RNA cannot. Signup and view all the answers

What does the term 'uncoating' refer to in the context of viral infection?

The release of the viral genome into the cytoplasm. Signup and view all the answers

What distinguishes 'early' viral gene products from 'late' viral gene products?

Early products are involved in genome replication; late products are involved in assembly. Signup and view all the answers

What process is described by the term 'budding' in the context of viral particle release?

Formation of new viral particles from the cell interior. Signup and view all the answers

What is the primary mechanism by which non-enveloped viruses enter a cell?

Endocytosis Signup and view all the answers

Which step in the viral replication cycle involves the removal of the capsid or envelope?

Uncoating Signup and view all the answers

What role do receptor molecules play in the virus replication cycle?

They facilitate the attachment of viruses to host cells. Signup and view all the answers

During which phase of the virus life cycle does the viral genome get delivered to the nucleus?

Uncoating Signup and view all the answers

What defines viropexis in the context of viral entry into host cells?

Phagocytosis of the virus by the host cell Signup and view all the answers

Why is the specificity of viral attachment important in viral pathogenesis?

It allows viruses to target specific tissues and cells. Signup and view all the answers

What happens to the infectivity of the parental virus during the uncoating process?

It is lost. Signup and view all the answers

Which type of viruses utilize a mechanism of entry that involves fusion with the host cell's plasma membrane?

Enveloped viruses Signup and view all the answers

What is the term for the phase where the virus loses infectivity after uncoating?

Eclipse phase Signup and view all the answers

Where does the replication of most RNA viruses occur?

Cytoplasm Signup and view all the answers

What must negative-sense RNA viruses do to produce proteins?

Use an enzyme to synthesize mRNA Signup and view all the answers

What type of RNA genome does a positive-sense RNA virus have?

Positive sense [5' to 3'] RNA Signup and view all the answers

What is a key characteristic of negative-sense RNA viruses?

They must convert their RNA to positive-sense RNA. Signup and view all the answers

What type of RNA viruses must supply their own RNA polymerase?

Negative-sense RNA viruses Signup and view all the answers

What occurs following the eclipse phase in the replication process of viruses?

Synthetic phase Signup and view all the answers

Which of the following correctly describes a characteristic of positive-sense RNA?

+ssRNA serves as mRNA directly. Signup and view all the answers

What is the role of the positive strand RNA viral genome in the infection process?

It acts as mRNA and directs protein synthesis. Signup and view all the answers

Which characteristic distinguishes positive-sense RNA viruses from negative-sense RNA viruses?

Positive-sense viruses interact directly with ribosomes. Signup and view all the answers

Which of the following viruses serves as an example of a negative-sense RNA virus?

Mumps virus Signup and view all the answers

What enzyme do retroviruses carry to convert their RNA genome into DNA?

Reverse transcriptase Signup and view all the answers

What is the primary role of the early proteins synthesized during viral replication?

They include polymerases and regulatory proteins. Signup and view all the answers

How does a negative-sense RNA virus replicate its genome?

With the aid of RNA-dependent RNA polymerase. Signup and view all the answers

What must be carried into the cell by negative-sense RNA viruses for proper replication?

RNA-dependent RNA polymerase. Signup and view all the answers

Which of the following statements about the naked positive strand viral genome is true?

It can initiate infection on its own. Signup and view all the answers

What do late viral gene products primarily encode?

Structural and packaging proteins Signup and view all the answers

Which step involves the conversion of single-stranded DNA into double-stranded DNA?

Replication of the genome Signup and view all the answers

What is the primary function of early viral gene products?

Catalytic actions in genome replication Signup and view all the answers

How are enveloped viruses primarily released from an infected cell?

Via budding from the plasma membrane Signup and view all the answers

What characterizes lytic viruses during the release phase?

They cause host cell death Signup and view all the answers

What role does virion RNA polymerase play in viral replication?

It transcribes negative RNA into positive RNA Signup and view all the answers

In which phase of viral synthesis is the transcription of late gene products initiated?

During genome replication Signup and view all the answers

What is the result of the irreversible combination of viral nucleic acid with the protein coat?

Assembly of new infectious virions Signup and view all the answers

What are the characteristics of viral mutants that influence their selection?

Use of host cell machinery Signup and view all the answers

What is a defective interfering particle (DIP)?

A variant with a critical portion of the genome lost Signup and view all the answers

What can result from mutations in essential viral genes?

Inactivation of the virus or drug resistance Signup and view all the answers

Which of the following is true regarding the transmission of viral nucleocapsid?

It involves direct fusion of infected cells Signup and view all the answers

How can viral vectors be utilized in gene therapy?

By using attenuated or defective viruses for gene delivery Signup and view all the answers

How do mutations in viral genomes lead to new viral strains?

They occur randomly and can affect various properties Signup and view all the answers

What is the outcome of cellular properties acting as selection pressures for viral mutants?

Elimination of weak viral strains Signup and view all the answers

What happens to the virus during the cell division of infected cells?

Gene copies are transmitted vertically to daughter cells Signup and view all the answers

Study Notes

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.

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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.