RNA Virus Replication

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Questions and Answers

Why are viruses considered obligate intracellular parasites?

  • They can reproduce through binary fission, similar to bacteria.
  • They possess all the necessary enzymes for protein synthesis and energy production.
  • They can survive and reproduce independently outside of a host cell.
  • They rely on the host cell's machinery for replication. (correct)

Which of the following best describes the Baltimore classification system for viruses?

  • It groups viruses based on their host cell type.
  • It organizes viruses according to their geographical distribution.
  • It classifies viruses based on their method of replication and genome type. (correct)
  • It categorizes viruses based on their size and shape.

How do viral attachment proteins (VAPs) facilitate viral infection?

  • By mediating the interaction between the virus and the target cell. (correct)
  • By producing antibodies against the virus.
  • By preventing the virus from entering the host cell.
  • By directly destroying the host cell's DNA.

Why are enveloped viruses more susceptible to inactivation outside of a host compared to non-enveloped viruses?

<p>The envelope is derived from the host cell membrane and is vulnerable to environmental factors. (C)</p> Signup and view all the answers

Which event characterizes the early phase of the viral replication cycle?

<p>Recognition of the target cell and penetration of the viral genome. (D)</p> Signup and view all the answers

How do interferons (IFNs) interfere with viral replication?

<p>By activating cellular mechanisms that inhibit viral replication. (A)</p> Signup and view all the answers

What is the eclipse period in the viral replication cycle?

<p>The period when the genome is uncoated, abolishing infectivity and identifiable structure. (D)</p> Signup and view all the answers

What is the significance of 'burst size' in viral infections?

<p>It represents the actual yield of infectious viruses produced per cell. (C)</p> Signup and view all the answers

What is the primary distinction between positive-sense (+) and negative-sense (-) RNA viruses in terms of replication?

<p>Positive-sense RNA can be directly translated into proteins, whereas negative-sense RNA must be transcribed into a positive-sense RNA first. (C)</p> Signup and view all the answers

Why is RNA-dependent RNA polymerase (RDRP) essential for the replication of RNA viruses?

<p>It synthesizes RNA from an RNA template, a process that does not occur in host cells. (C)</p> Signup and view all the answers

How does reverse transcriptase contribute to the replication of retroviruses?

<p>By synthesizing DNA from an RNA template. (A)</p> Signup and view all the answers

What is the role of integrase in retroviral replication?

<p>To integrate the viral cDNA into the host cell's genome. (B)</p> Signup and view all the answers

In retroviruses, what is the significance of carrying two tRNA molecules?

<p>They serve as primers for reverse transcriptase during cDNA synthesis. (C)</p> Signup and view all the answers

Why are DNA viruses that do not encode their own enzymes primarily limited to infecting cells undergoing mitosis?

<p>Host DNA polymerases are readily available during mitosis to replicate the viral genome. (D)</p> Signup and view all the answers

What is the function of viral proteases in the replication of positive-sense RNA viruses?

<p>To cleave the large polyprotein into individual functional proteins. (D)</p> Signup and view all the answers

How does the binding of mRNA to the ribosome typically initiate the process of viral protein synthesis?

<p>Through the recognition of a 5' cap or an internal ribosome entry site (IRES). (C)</p> Signup and view all the answers

Why does Herpes Simplex virus have more freedom in target cell selection compared to some other DNA viruses?

<p>It encodes its own primases and polymerases. (C)</p> Signup and view all the answers

How does the presence of a replicative intermediate relate to the host response during viral infection?

<p>Strong inducer of host response (D)</p> Signup and view all the answers

In the context of viral replication, what is budding and its significance?

<p>The process by which some viruses exit the host cell without killing it (D)</p> Signup and view all the answers

How can Adenoviruses and Papillomaviruses alter cell growth?

<p>By inhibiting growth-inhibitory proteins. (C)</p> Signup and view all the answers

Why is it essential for negative-sense RNA viruses to bring their own RNA-dependent RNA polymerase (RDRP) into the host cell?

<p>Host cells are incapable of synthesizing RNA from an RNA template. (D)</p> Signup and view all the answers

What role do viral enhancers and promoter elements play in the context of DNA virus replication?

<p>Hijack host cell polymerases for viral replication (D)</p> Signup and view all the answers

What typically happens to the host cell after the envelopment process?

<p>The host cell can continue to produce new virions. (B)</p> Signup and view all the answers

What makes the capsids of non-enveloped viruses especially important?

<p>Capsids act as protection for viruses without an envelope. (D)</p> Signup and view all the answers

What is a major limitation for many viruses in terms of their host cell selection?

<p>Many viruses can only target cells undergoing mitosis (C)</p> Signup and view all the answers

Which of the following viruses integrates into DNA to replicate?

<p>Retro (C)</p> Signup and view all the answers

Which of the characteristics matches with RNA (+)?

<p>Enter target. (A)</p> Signup and view all the answers

Considering viral infections, what is the period of latency characterized by?

<p>A period when the virus is present in the host but is not actively replicating (D)</p> Signup and view all the answers

Why is knowledge of viral structure important in developing antiviral treatments?

<p>Allows scientists target specific interactions to block viral infection. (D)</p> Signup and view all the answers

In what scenario are virions only transmitted through fluids?

<p>Aqueous (D)</p> Signup and view all the answers

How can viruses be destroyed?

<p>Antibodies (C)</p> Signup and view all the answers

What are the differences between the helical and icosahedral shapes?

<p>Helical is rod and Icosahedral is spherical. (D)</p> Signup and view all the answers

What can be composed of Virions?

<p>Nucleic acids and protein coats. (B)</p> Signup and view all the answers

Which phase of the viral replication cycle culminates in the release of new virions through lysis or budding?

<p>The late phase (B)</p> Signup and view all the answers

Besides specific structural features, what else helps the virus by speeding up cell growth?

<p>Papilloma (D)</p> Signup and view all the answers

How do non-enveloped viruses typically initiate infection, considering their structural properties?

<p>By utilizing viral attachment proteins (VAPs) on the capsid to mediate direct interaction with the host cell's surface receptors. (D)</p> Signup and view all the answers

In the context of viral replication, what is the fundamental difference in how positive-sense (+) RNA viruses and negative-sense (-) RNA viruses initiate protein synthesis upon entering a host cell?

<p>Positive-sense RNA viruses can be directly translated into proteins, whereas negative-sense RNA viruses must first be transcribed into a positive-sense RNA by RNA-dependent RNA polymerase (RDRP). (A)</p> Signup and view all the answers

Which aspect of viral infection does the "eclipse period" specifically refer to?

<p>The time after uncoating and before the appearance of new, infectious virions in the cell. (B)</p> Signup and view all the answers

How do interferons (IFNs) combat viral infections at the cellular level?

<p>By inducing the production of proteins that inhibit viral protein synthesis and degrade viral RNA. (A)</p> Signup and view all the answers

How does the presence of a replicative intermediate typically affect the host's response to a viral infection?

<p>Replicative intermediates, not normally found in cells, serve as strong inducers of the host's immune response. (D)</p> Signup and view all the answers

Flashcards

Viruses

Obligate intracellular structures that reproduce via the assembly of smaller components instead of replicating with binary fission.

Virions

Viral particles composed of nucleic acid (DNA or RNA) plus a nucleocapsid, and sometimes a membraneous envelope plus essential enzymes

Viral attachment proteins (VAPs)

These mediate the interaction of the virus with the target cell, and destruction makes the virus unable to infect

Capsid

Rigid, resistant structure, often transmitted via the oral-fecal route

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Envelope (viral)

Membraneous structure only maintained in aqueous solutions. And are very vulnerable to drying acids and detergents

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Early phase (viral replication)

Virus recognizes target cell, attaches, penetrates, & uncoats its genome

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Late Phase (viral replication)

Encompasses genome replication, macromolecular synthesis, viral assembly, and release

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Interferons

Proteins that interfere with viral replication in an endogenous system

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Latent period

The period in which no extracellular infections or viruses are detected

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Burst size

Yield of infectious viruses per cell

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DNA viruses replication

Requires DNA-dependent-DNA-Polymerases, dNTPs, other enzymes to replicate. Hijack host cell polymerase

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RNA Viruses (+)

double-stranded RNA virus genome codes for RNA-dependent RNA Polymerases

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RNA viruses (-)

Enveloped fusion entry and must bring in own RDRP

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Retroviruses

carry an RNA-dependent-DNA polymerase (reverse transcriptase)

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Study Notes

Replication of RNA Viruses (+) and (-)

  • Double-stranded RNA replication produces a replicative intermediate, a strong inducer of host response
  • RNA viruses (+) must code for RNA-dependent RNA polymerases (RdRp)
  • (+) strand RNA is sufficient to initiate itself

RNA Virus (-) Replication: Rhabdoviridae Example

  • Enveloped viruses use fusion entry
  • Must bring its own RdRp, consisting of L protein and Ns (nucleoprotein)
  • RdRp synthesizes smaller positive strands from the template strand (N, G, M, NS, etc.)
  • These strands then code for all necessary enzymes
  • Enters the target cell and does not have to enter the nucleus
  • (+) strand is then copied into (-) strand to serve as template for more (+) strands (replication intermediate)
  • The new, (-) strands exit the cell by budding and release

Retroviruses Example: (+) RNA into DNA (VI)

  • Provides no means for RNA replication in the cytoplasm
  • Has a (+) strand RNA genome and carries 2 copies of the genome
  • Also carries 2 tRNAs (serve as primers) and RNA-dependent-DNA polymerase (reverse transcriptase)
  • Complementary DNA (cDNA) is synthesized in the cytoplasm, travels to the nucleus, and integrates into the host genome
  • Integrate cDNA using enzyme integrase into the host's DNA as proviral DNA
  • HIV targets helper T cells by attaching to CD4 with CCR5, causing the envelope to fuse
  • Host cell RNA polymerase transcribes proviral DNA into mRNA, which is then translated and releases genomic RNA
  • Eukaryotic ribosomes bind to mRNA and can make only one continuous protein
  • (+) RNAs are translated into one big "poly protein," then cleaved by proteases

DNA Virus: Herpes Simplex

  • Encodes its own primases and polymerases, allowing more freedom in target cell selection
  • It can perform cell-to-cell release, avoiding antibodies
  • Enveloped virions uncoat in the nucleus
  • They exit the nucleus in a capsid and the cell in an envelope

Viral Protein Production

  • All viruses rely on host cell ribosomes, tRNA, and mechanisms for post-translational modifications to make their proteins
  • mRNA binding to the ribosome is mediated by a 5' cap or internal ribosome entry sequence (IRES)

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