Viral Replication Processes
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Questions and Answers

What is the role of reverse transcriptase in retroviruses?

Reverse transcriptase converts positive sense ss RNA into negative sense ss DNA.

Distinguish between early and late viral proteins in the synthesis process.

Early proteins are enzymes required for genome replication, while late proteins are structural proteins for progeny viruses.

How does the viral assembly process occur?

Viral nucleic acid is incorporated into a capsid to form a mature virion.

What is the final step in the replication cycle of viruses?

<p>The final step is the release of completed viruses from the host cell.</p> Signup and view all the answers

What distinguishes the release of enveloped viruses from nonenveloped viruses?

<p>Enveloped viruses release through budding, whereas nonenveloped viruses often cause cell lysis.</p> Signup and view all the answers

In what way can herpes virus differ in its release mechanism compared to other enveloped viruses?

<p>Herpes virus buds from the nuclear membrane instead of the outer cell membrane.</p> Signup and view all the answers

Explain the transcription process of viral mRNA in the context of reverse transcribed RNA viruses.

<p>The negative sense ss DNA produced is transcribed into viral mRNA by host cell RNA polymerase.</p> Signup and view all the answers

Why do late viral proteins typically appear after genome replication?

<p>Late viral proteins are mainly structural proteins that assemble the new virions after the genome has been replicated.</p> Signup and view all the answers

What role do lysosomal enzymes play in the uncoating process of most viruses?

<p>Lysosomal enzymes degrade the protein of the viral capsid, facilitating uncoating.</p> Signup and view all the answers

How does the uncoating process differ for poxvirus compared to other viruses?

<p>Poxvirus relies on a specific enzyme encoded by the viral DNA for uncoating, while other viruses may utilize host cell enzymes.</p> Signup and view all the answers

What is the significance of transcribing specific mRNAs from viral nucleic acid?

<p>Transcribing specific mRNAs is essential for the successful expression and duplication of the viral genetic information.</p> Signup and view all the answers

How do DNA viruses produce their mRNA?

<p>DNA viruses use the host's RNA polymerase to transcribe mRNA directly from their viral DNA.</p> Signup and view all the answers

What mechanism do double-stranded RNA viruses use to form mRNA?

<p>Double-stranded RNA viruses transcribe one strand into mRNA using their own viral polymerase.</p> Signup and view all the answers

What is the significance of the CD4 receptor in the context of viral attachment?

<p>The CD4 receptor is crucial for HIV's attachment to target cells, determining the virus's ability to infect those cells.</p> Signup and view all the answers

What distinguishes positive-sense single-stranded RNA viruses in mRNA production?

<p>Positive-sense ss RNA viruses can directly use their RNA genome as mRNA without further transcription.</p> Signup and view all the answers

How do nonenveloped viruses typically penetrate host cells?

<p>Nonenveloped viruses enter host cells through endocytosis, specifically a process known as viropexis.</p> Signup and view all the answers

What must negative-sense single-stranded RNA viruses do before they can produce mRNA?

<p>Negative-sense ss RNA viruses must transcribe their RNA into a positive-sense strand, which can then act as mRNA.</p> Signup and view all the answers

What type of polymerase do RNA viruses require for mRNA synthesis?

<p>RNA viruses require their own viral polymerase for transcribing mRNA from their RNA templates.</p> Signup and view all the answers

Describe the uncoating process in viral replication.

<p>Uncoating is the process where the viral nucleic acid is separated from its protein core, allowing for replication.</p> Signup and view all the answers

Where does genome replication generally occur for DNA viruses, and what is the exception?

<p>Most DNA virus genome replication occurs in the nucleus, except for Poxviruses, which replicate in the cytoplasm.</p> Signup and view all the answers

What distinguishes the replication location of RNA viruses from that of retroviruses?

<p>RNA viruses generally replicate in the cytoplasm, while retroviruses replicate in the nucleus.</p> Signup and view all the answers

Explain the role of viral envelope proteins during the entry of enveloped viruses.

<p>Viral envelope proteins facilitate the fusion of the virus with the host's plasma membrane, allowing the capsid to enter the cell.</p> Signup and view all the answers

Why do RNA viruses have a higher mutation rate compared to DNA viruses?

<p>RNA viruses have a higher mutation rate due to their error-prone RNA polymerase during replication.</p> Signup and view all the answers

What determines the host range of a virus?

<p>The specific surface receptors on a cell determine the host range, as viruses can only infect cells with compatible receptors.</p> Signup and view all the answers

What is the Eclipse period in the viral replication cycle?

<p>The Eclipse period is the time during which no virus is found inside the host cell, starting from the penetration stage until mature daughter virions appear.</p> Signup and view all the answers

How long does a single cycle of viral replication typically take?

<p>A single cycle of viral replication typically takes about 15–30 hours.</p> Signup and view all the answers

Define the latent period in the context of viral infection.

<p>The latent period is the time from the onset of infection to the appearance of the virus extracellularly.</p> Signup and view all the answers

What is meant by cytopathic effect (CPE) in viral infections?

<p>Cytopathic effect (CPE) refers to the alterations in cell morphology and function resulting from viral infection, culminating in cell lysis and death.</p> Signup and view all the answers

List the first three steps in the viral replication cycle.

<p>The first three steps in the viral replication cycle are recognition of the target cell, attachment of the virus particle to the cell surface, and penetration into the host cell.</p> Signup and view all the answers

What happens during the uncoating stage of viral replication?

<p>During the uncoating stage, the virus sheds its outer layers and capsid to release its genetic material into the host cell.</p> Signup and view all the answers

Why are viruses considered obligatory intracellular parasites?

<p>Viruses are considered obligatory intracellular parasites because they lack the machinery to synthesize their nucleic acids or proteins independently and rely on the host cell's metabolic processes.</p> Signup and view all the answers

What role does the host cell's metabolic machinery play in viral replication?

<p>The host cell's metabolic machinery is utilized by viruses to synthesize their nucleic acids and proteins necessary for replication.</p> Signup and view all the answers

Flashcards

Viral Replication Cycle

The series of steps a virus takes to multiply within a host cell.

Eclipse Period

The time during viral replication when no intact virus particles can be detected within the cell.

Latent Period

The time from the moment of infection to the appearance of newly formed viruses outside the cell.

Cytopathic Effect (CPE)

Visible changes in the host cell caused by viral infection, leading to cell damage and death.

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Obligatory Intracellular Parasites

Viruses require a host cell to replicate and survive.

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Viral Genome

The genetic material of a virus, containing instructions for its replication.

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Transcription

The process of copying the viral genetic information into mRNA.

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Biosynthesis

The process where the host cell's machinery is used to create viral proteins and nucleic acids.

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Uncoating

The process where a virus's protein coat is removed, exposing its genetic material for replication within the host cell.

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Lysosomal enzymes

Enzymes present in host cells that can break down viral capsid proteins during uncoating.

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Viral DNA-encoded enzyme

A specific enzyme synthesized by the virus after infection, responsible for uncoating in some viruses.

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Gene expression in DNA viruses

DNA viruses utilize the host's RNA polymerase to create mRNA directly from their DNA genome.

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Gene expression in RNA viruses

RNA viruses use various methods to create mRNA, as host cell polymerases cannot directly transcribe RNA.

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Double-stranded RNA viruses

These viruses contain two strands of RNA. One strand is transcribed into mRNA by the virus's own RNA polymerase.

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Single-stranded RNA of positive polarity

This type of RNA virus has a sequence directly usable as mRNA, so its genome can be used directly.

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Single-stranded RNA of negative polarity

This type of RNA virus requires transcription into a positive strand before it can act as mRNA.

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Viral Attachment

The initial step of viral infection, where the virus binds to specific receptors on the host cell surface. This receptor specificity determines the host range of the virus.

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Viral Receptors

Specific molecules on the surface of a host cell that viruses recognize and bind to for attachment. These receptors determine which cells a virus can infect.

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Endocytosis

A cellular process where a cell engulfs material from the outside environment by forming a vesicle. This is how non-enveloped viruses enter the cell.

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Viropexis

Another name for endocytosis, specifically referring to the entry of a virus into a cell by this process.

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Fusion

The merging of the viral envelope with the host cell membrane, allowing the capsid to enter the cell. This is how enveloped viruses enter.

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Early Proteins

Proteins produced by the virus early in the infection cycle, often involved in replicating viral DNA or RNA.

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Late Proteins

Proteins produced by the virus later in the infection cycle, often involved in assembling new viral particles.

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Reverse Transcriptase

An enzyme used by retroviruses to convert their RNA genome into DNA. This DNA can then be integrated into the host's genome.

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Early Viral Proteins

Proteins synthesized during the early stages of viral infection, often involved in replication of the viral genome.

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Late Viral Proteins

Proteins synthesized after genome replication. These often form the structural components of the new viral particles.

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Assembly (In Viral Replication)

The process where newly synthesized viral components (DNA/RNA and proteins) come together to form new virus particles (virions).

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Release (Viral Replication)

The process where mature viruses leave the host cell. This can happen through budding or rupture of the cell membrane.

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Non-Enveloped Virus Release

These viruses release from the host cell by causing the cell membrane to rupture, often leading to the death of the cell.

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Enveloped Virus Release

These viruses release from the host cell by budding, wrapping themselves in a portion of the cell membrane. This can sometimes keep the host cell alive.

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Viral Growth Cycle

The complete series of events that a virus undergoes inside a host cell, from attachment and entry to replication, assembly, and release.

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Study Notes

Viral Replication

  • Viral replication occurs in two ways: a growth curve and a stepwise description.
  • The growth curve shows the amount of virus produced over time after infection.
  • The first event is the disappearance of the virus particles; the viral nucleic acid remains and functions within the cell.
  • The eclipse period is the time when no virus is found inside the cell, from penetration to mature virion appearance.
  • A single infected cell can produce many daughter virions.
  • The replication cycle takes 15-30 hours.
  • The latent period is the time from infection to the appearance of extracellular virus.
  • At the end of the latent period, the cell morphology changes, with impaired function. This is known as the cytopathic effect (CPE).

Steps in Viral Replication

  • Viruses utilize host cell machinery for replication as they lack their own.
  • Replication involves a series of steps:
    • Recognition of the target cell
    • Attachment of the virus to the cell surface
    • Penetration of the host cell
    • Uncoating of the virus
    • Biosynthesis, including transcription (mRNA synthesis), translation (protein synthesis), replication (viral nucleic acid synthesis), and protein synthesis of late proteins
    • Assembly of the virus components
    • Budding of enveloped viruses
  • DNA viruses replicate primarily in the nucleus, except poxviruses, whilst RNA viruses primarily replicate in the cytoplasm; retroviruses and influenza viruses replicate in the nucleus.

Early Events (Attachment, Penetration, and Uncoating)

  • Adsorption (Attachment): Virus particles attach to specific receptors on the host cell surface, determining host range.
  • Penetration: Non-enveloped viruses enter via endocytosis (viropexsis). Enveloped viruses fuse their membranes with the host cell membrane, releasing their capsid into the cytoplasm.
  • Uncoating: Viral nucleic acid separates from its protein coat. This is accomplished by lysosomal enzymes in some viruses or host cell enzymes in others.

Middle Events (Gene Expression and Genome Replication)

  • Gene expression for successful viral component expression and duplication requires viral mRNA transcription.
  • DNA viruses transcribe mRNA directly from viral DNA using host RNA polymerases.
  • RNA viruses must produce their mRNAs since host polymerases do not work with RNA. Different strategies exist based on the RNA strand polarity (positive or negative).
    • Double-stranded RNA viruses: one strand is transcribed into mRNA by viral polymerase.
    • Single-stranded RNA (positive sense): RNA itself acts as mRNA.
    • Single-stranded RNA (negative sense): The virus carries its RNA-dependent RNA polymerase that transcribes the genome to form mRNA.
    • Reverse-transcribed RNA viruses: The RNA is initially transcribed into DNA by reverse transcriptase and then into mRNA by host enzymes.

Late Events (Assembly and Release)

  • Assembly: Viral components assemble into mature virions within the cell.
  • Release:
    • Non-enveloped viruses rupture the host cell's membrane, leading to cell death.
    • Enveloped viruses bud from the cell membrane. This is a common mechanism for many viruses, including HIV & Influenza Virus. Some viruses (e.g., Herpes) bud from the nuclear membrane. Release doesn't always kill the host cell.

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Description

Explore the complex processes involved in viral replication, including both growth curves and stepwise descriptions. Learn about the stages of infection, from the eclipse period to the cytopathic effect, and the crucial role of host cell machinery in the replication cycle.

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