Introduction to Viruses and Their Discovery
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Questions and Answers

What role do scaffolding proteins play during the assembly of certain viruses?

  • They temporarily assist in the construction of the procapsid before being removed. (correct)
  • They are crucial for self-assembly in a test tube.
  • They form the final structure of the virion.
  • They help in the budding process of enveloped viruses.

    • Which mechanism is commonly used by enveloped viruses to acquire their viral envelopes?

  • Synthesis of new membranes exclusively.
  • Direct lysis of host cells to expose the envelope.
  • Modification of host cell membranes through direct synthesis.
  • Budding through host cell membranes by incorporation of virus proteins. (correct)

    • What distinguishes the egress mechanism of bacteriophages from that of animal viruses?

  • Bacteriophages initiate cell lysis, while animal viruses release virions over time. (correct)
  • Both types of viruses require vesicle transportation to exit the cell.
  • Bacteriophages bud from the plasma membrane, while animal viruses lyse their host cells.
  • Animal viruses typically lyse their host cells, while bacteriophages are released slowly.

    • Which of the following statements best describes self-assembly in viruses?

    <p>Self-assembly can occur in vitro without the need for additional genetic information.</p> Signup and view all the answers

    What is a key feature of the viruses that rely on directed assembly for virion construction?

    <p>They require an internal cellular environment for assembly and maturation.</p> Signup and view all the answers

    What is the role of packaging signals in viral genome selection during assembly?

    <p>They allow viruses to discriminate between genomic and non-genomic RNAs.</p> Signup and view all the answers

    Which statement best describes the function of procapsid in virion assembly?

    <p>Procapsid stores viral RNA and may be modified to form the mature capsid.</p> Signup and view all the answers

    What mechanism do certain viruses use for egress from infected cells?

    <p>Viruses exploit cellular exocytosis pathways without altering cell function.</p> Signup and view all the answers

    Which characteristic of viral membrane formation distinguishes enveloped viruses from non-enveloped viruses?

    <p>Enveloped viruses acquire their membrane from host cell membranes.</p> Signup and view all the answers

    What is a key factor that enables some viruses to self-assemble from purified components?

    <p>The simplicity of their structure, composed of one or few protein species.</p> Signup and view all the answers

    Study Notes

    General Introduction

    • Viruses infect all life forms (eukaryotes, prokaryotes, plants, fungi)
    • Viruses cause diseases, from common colds to lethal diseases like rabies, and contribute to cancer development.
    • Viruses impact societal well-being
    • Viruses have diverse genomes (RNA or DNA, single-stranded or double-stranded)
    • Viruses are very small (less than 200 nanometers in diameter)
    • Viruses can only replicate inside a host cell
    • Viruses don't contain ribosomes (needed for protein synthesis)
    • New virions are created through replication inside the host cell
    • Viruses rely on host cells for building blocks (amino acids, nucleosides) and energy (ATP)
    • Viruses modify host cells to enhance replication efficiency

    Discovery of Viruses

    • Beijerinck and Ivanovski independently discovered evidence of very small infectious agents
    • Extracts from diseased plants with tobacco mosaic virus passed through filters
    • Agents remained infectious after transfers, eliminating toxins as the cause.
    • Beijerinck named them "viruses"

    Viruses Living or Non-living?

    • Ongoing debate over whether viruses are living or non-living
    • Viruses have genes and replicate when infecting cells, suggesting a living quality
    • Outside host cells, viruses exist as inert particles (virions), resembling non-living entities
    • Strict definitions of life classify viruses as non-living

    Origin of Viruses

    • Four theories, not mutually exclusive:
      • Regressive: remnants of cellular organisms
      • Progressive: escaped nucleic acids from cells
      • Symbiontic: intracellular microorganisms
      • Virus-first: independent origin from self-replicating molecules.

    Classification and Nomenclature

    • International Committee on Taxonomy of Viruses (ICTV) established rules
    • Nomenclature based on genome type (DNA or RNA, single- or double-stranded) and replication methods.
    • Baltimore classification groups viruses by genome type
    • dsDNA
    • ssDNA
    • dsRNA
    • (+)ssRNA
    • (-)ssRNA
    • ssRNA with DNA intermediate
    • dsDNA-RT

    Structure of Viruses

    • Virus particles (virions) outside host cells
    • Virion structure: genome enclosed in capsid, sometimes with an outer envelope
    • Capsids: protein structure, helical or icosahedral symmetry
    • Envelopes: lipid membrane containing glycoproteins
    • Various genome composition (dsDNA, ssRNA, dsRNA)

    Viral Infection

    • Viral aim is to replicate within a host cell
    • Seven steps in viral replication cycle
      • Attachment to cell
      • Entry into cell
      • Transcription to mRNA
      • Translation to proteins
      • Genome replication
      • Assembly of virions
      • Exit from cell
    • Different mechanisms for attachment and entry (naked vs enveloped viruses)
    • Crucial steps in viral replication: endocytosis, and fusion with host cell membranes
    • Viral genomes can be replicated through various mechanisms to DNA/RNA/RNA-based.

    Transport of Newly Synthesized Viral Proteins

    • Viral proteins transported to various locations within infected cells (nucleus, etc.)
    • Signal sequences direct proteins to their destinations

    Virus Genome Replication

    • DNA viruses replicate directly to DNA
    • RNA viruses replicate through RNA or DNA intermediate
    • Classification systems based on (+) and (-) strands
    • Replication strategies (theta, sigma) and mechanisms

    Assembly and Release (Virion Exit)

    • Assembly process (and procapsids)
    • How the virus leaves the infected host cell, envelope acquisition, and egress mechanisms
    • Importance of proteins in viral assembly and release

    Outcomes of Infection and Immune Responses

    • Productive infection: new virions exit the host cell.
    • Latent infection: virus genome persists.
    • Abortive infection: no virus replication.
    • Defective viruses: require helper virus for replication
    • Host defenses have developed countermeasures to viruses

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    Viruses - I - Supplementary Material PDF

    Description

    Explore the fascinating world of viruses, their structure, and their role in disease across diverse life forms. Learn about the pioneering discoveries of Beijerinck and Ivanovski that led to the understanding of these microscopic agents. This quiz will provide insights into how viruses replicate and interact with host cells.

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