Viral Replication Cycle Lecture 3
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Questions and Answers

Which type of virus has a replication cycle that starts with its RNA genome being directly used as mRNA?

  • Single stranded RNA viruses of positive sense (correct)
  • Retroviruses
  • Single stranded RNA viruses of negative sense
  • Double stranded RNA viruses

    • Which type of virus relies on a viral RNA-dependent-RNA polymerase to transcribe its mRNA?

  • Double stranded RNA viruses (correct)
  • Single stranded RNA viruses of negative sense (correct)
  • Single stranded RNA viruses of positive sense
  • Retroviruses

    • What is the key enzyme responsible for integrating retroviral DNA into the host cell's genome?

  • RNA-dependent-RNA polymerase
  • Integrase (correct)
  • DNA-dependent-RNA polymerase
  • Reverse transcriptase

    • Which of the following viruses replicates entirely within the cytoplasm?

    <p>Poxviruses</p> Signup and view all the answers

    Which viral group utilizes its own RNA-dependent-RNA polymerase for the initial steps of viral replication?

    <p>Double stranded RNA viruses</p> Signup and view all the answers

    What is the unique characteristic of retroviruses that gives them their name?

    <p>They transcribe their RNA genome into DNA.</p> Signup and view all the answers

    Which viral group requires a host cell DNA-dependent-RNA polymerase for its replication?

    <p>Most DNA viruses</p> Signup and view all the answers

    Which of these steps is NOT involved in the replication cycle of a retrovirus?

    <p>Transcription of viral DNA into RNA using viral RNA-dependent-RNA polymerase</p> Signup and view all the answers

    Which of the following statements is FALSE regarding viral attachment and penetration?

    <p>Viruses typically utilize a single specific cellular receptor for attachment.</p> Signup and view all the answers

    Which viral protein is involved in the attachment of influenza viruses to host cells?

    <p>Hemagglutinin</p> Signup and view all the answers

    What is the primary role of the chemokine receptor CCR5 in HIV infection?

    <p>It acts as a co-receptor, facilitating viral entry into CD4+ T cells.</p> Signup and view all the answers

    The process of uncoating the viral genome is essential for viral replication. Which of the following statements accurately describes the process?

    <p>The viral genome is exposed through the action of cellular lysosomes, triggered by a drop in pH.</p> Signup and view all the answers

    How does the pH change during uncoating contribute to the release of the viral genome?

    <p>The acidic environment within lysosomes disrupts the interaction between the viral genome and its associated proteins.</p> Signup and view all the answers

    Which of the following statements best describes the term 'cell tropism' in the context of viral infection?

    <p>The ability of a virus to infect only specific cell types.</p> Signup and view all the answers

    Which of the following statements is TRUE regarding the role of viral proteins in attachment and penetration?

    <p>Viral proteins may interact with cellular receptors to trigger the engulfment of the virus by pinocytosis.</p> Signup and view all the answers

    Which of the following best describes the significance of the viral genome being exposed during uncoating?

    <p>It enables the viral genome to replicate and produce new viral proteins.</p> Signup and view all the answers

    What is the main function of virus-encoded polymerase?

    <p>To make many copies of the parental genome</p> Signup and view all the answers

    What is the process by which progeny virions are released from the cell?

    <p>By rupture of the cell membrane or budding through the outer cell membrane</p> Signup and view all the answers

    What is the result of recombination between two different viruses infecting the same cell?

    <p>Progeny virus carries trait not found in either parent</p> Signup and view all the answers

    What is the main difference between recombination and reassortment?

    <p>Reassortment results in a higher frequency of gene exchange than recombination</p> Signup and view all the answers

    What is the process of genetic reactivation also known as?

    <p>Marker rescue</p> Signup and view all the answers

    What is the result of complementation between two viruses?

    <p>The genotype of neither virus changes</p> Signup and view all the answers

    What is the process by which viruses with segmented genomes exchange genetic material?

    <p>Reassortment</p> Signup and view all the answers

    What is the reason for recurrent influenza epidemics?

    <p>Reassortment of influenza virus RNA segments</p> Signup and view all the answers

    Study Notes

    Viral Replication Cycle

    • Stages of viral growth cycle:
      • Attachment and penetration
      • Uncoating of the viral genome
      • Early viral mRNA synthesis
      • Early viral protein synthesis
      • Viral genome replication
      • Late viral mRNA synthesis
      • Late viral protein synthesis
      • Progeny virion assembly
      • Virion release from the cell

    Attachment and Penetration

    • Specific viruses use specific cellular receptors on the cell surface, determining cell tropism and disease pathogenesis
    • Examples: HSV-1 attaches to fibroblast growth factor receptor, Influenza viruses use haemagglutinin (HA) protein to attach to sialic acid-containing oligosaccharides on the cell surface
    • Viruses may use more than one cell receptor, such as HIV using CD4 receptor and chemokine receptor CCR5 as a co-receptor
    • Most viruses use multiple receptors on the cell surface in a sequential binding process
    • Viral particle penetration occurs through engulfment in a pinocytotic vesicle for enveloped viruses, or fusion of lipid envelope with cell membrane

    Uncoating of the Viral Genome

    • Before replication, the viral genome must be exposed by removal of associated viral proteins
    • Mediated by merging of endocytosed viral particle with cellular lysosomes
    • Resulting drop in pH dissociates viral genome from its binding protein
    • Rupture of vesicle deposits inner core of virus into cytoplasm

    Early Viral mRNA Synthesis

    • In DNA viruses, replication occurs in the nucleus, using host cell DNA-dependent-RNA polymerase to synthesize mRNA (except poxviruses, which replicate in the cytoplasm)
    • In RNA viruses, entire replicative cycle occurs in the cytoplasm, except retroviruses and influenza viruses, which have important replicative steps in the nucleus
    • RNA viruses have different strategies for synthesizing mRNA:
      • Single-stranded RNA viruses of positive sense use their RNA genome directly as mRNA
      • Single-stranded RNA viruses of negative sense transcribe their mRNA using the negative strand as a template
      • Double-stranded RNA viruses convert the -ve RNA strand into a complementary RNA to be used as mRNA
      • Retroviruses have single-stranded RNA of positive sense, transcribed into double-stranded DNA by RNA-dependent DNA polymerase (reverse transcriptase)

    Early Viral Protein Synthesis

    • Viral mRNA is translated by host cell ribosomes into viral proteins, including early proteins (enzymes required for replication of viral genome) and late proteins (structural proteins of progeny viruses)

    Viral Genome Replication

    • Most viruses make virus-encoded polymerase that make many copies of the parental genome, becoming the genome of the progeny virions

    Late Viral mRNA Synthesis and Protein Synthesis

    • Occur after replication of the genome
    • Result in the production of late proteins, including structural proteins of progeny viruses

    Progeny Virion Assembly and Release

    • Progeny virion assembly involves packaging the viral nucleic acid within the capsid proteins (late proteins)
    • Virion release from the cell occurs through either rupture of the cell membrane and release of mature virus particles or release of viruses by budding through the outer cell membrane in enveloped viruses

    Virus Genetics and Interactions Among Viruses

    • Interactions include recombination and reassortment, genetic reactivation (marker rescue), complementation, phenotypic mixing, and interference
    • Recombination: crossing over between the two chromosomes of different viruses infecting the same cell, resulting in progeny virus carrying traits not found in either parent
    • Reassortment: exchange of genetic material between viruses with segmented genomes, resulting in higher frequency of gene exchange than recombination
    • Genetic reactivation (marker rescue): between the genome of active virion and inactivated virus genome, rescuing certain markers of the inactivated parent and appearing in the progeny
    • Complementation: one virus provides a gene product in which the second is defective, allowing the second virus to grow

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    Viral Replication Cycle PDF

    Description

    This quiz covers the stages of viral growth cycle, including attachment, penetration, uncoating of the viral genome, and more. Learn about the different steps involved in viral replication.

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