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Debate on Viruses as Living or Non-Living

  • Ongoing discussion about the classification of viruses as living or non-living entities.

  • Key characteristics of viruses include:

    • Protein coat (Capsid): The protective outer shell made up of protein.

    • Nucleic acid (DNA or RNA): The genetic material inside the capsid, which varies by virus type.

Structure of Viruses

  • Basic structure of viruses:

    • Viruses possess a capsid consisting of protein subunits called capsomers.

    • They can either be enveloped or non-enveloped.

    • Examples:

    • Enveloped viruses have a lipid layer surrounding the capsid (e.g., HIV).

    • Phages are viruses specifically targeting bacteria (e.g., T4 phage targeting E. Coli).

Types of Capsids

  • Viruses exhibit various shapes for their capsids:

    • Polyhedral structure: A common geometric shape for many viruses.

    • Cubical structure: Another typical geometric formation.

    • Helical structure: The first virus type discovered.

    • Different viruses may possess different shapes and sizes reflecting their structural diversity.

Viral Composition and Mechanism

  • Viruses may contain either single-stranded or double-stranded nucleic acids.

  • They can have circular or linear forms of nucleic acids, with varying complexities.

  • Viruses inject their nucleic acids into host cells:

    • During the extracellular state, viruses survive outside of cells without reproductive capabilities.

    • Upon entering the host (intracellular state), they may shed their protein coat and inject only the nucleic acids.

Viral Classification Based on Nucleic Acid

  • Virus classification can include:

    • Type of nucleic acid: DNA or RNA.

    • Structure of the nucleic acid (single-stranded, double-stranded).

    • Presence or absence of an envelope.

  • Genome simplicity: Viral genomes are minimal, often encoding only the essential proteins required for replication (capsid proteins and nucleic acids).

Viral Replication Process

  • Lytic Cycle: rapid replication; host dies

    1. Attachment: the bacteriophage attaches to host cell (E.coli) using tail fibers

    2. Entry: phage injects DNA into host

      • capsid remains outside

      • Host cell DNA is degraded

    3. Synthesis: The host enzymes, ribosomes, organelles, are used to transcribe/translate viral DNA

      • build parts for new viruses

    4. Assembly: viruses are assembled & prepare to leave cell

    5. Release: host lysis when viruses leave cell & enter envelop

Lysogenic Cycles

Lysogenic Cycle: viral genome is intergrated into host, cell survives until induction

  1. attachment & entry = same as lytic

  2. Intergradation: viral DNA inserts into host genome turning it into a prophage

  3. Replication: host is still alive, virus remains dormant, host replicates + behaves as normal

Induction: an env. Stress (stress/ uv radiation) causes probates to enter lytic cycle

  • DNA from the host will be degraded, new virus synthesized, cell lyse viruses are released

Burst time

  • time is takes between entry & release

    • low burst time = virus is faster

Animal viruses

  • Only attacks animal cells

  • Has envelope

    • budding

Animal virus entry

  1. Direct penetration: virus injects genetic material into cell, leaving capsid outside

    • similar to bacteriophage

  2. Membrane fosion: envelope fuses with animal cell membrane! Capsid dissolves

  3. Endocytosis: host cell engulfs virus & uncoats DNA

  • host “pulls” virus in

Immune Response to Viral Infections

  • Fever elevates body temperature, enhancing immune response effectiveness.

  • Antibodies bind to viral proteins to prevent receptor binding, inhibiting infection.

  • Viruses have varying lifespans outside hosts; enveloped viruses tend to have shorter lifespans compared to non-enveloped viruses.

  • Electron microscopy is essential for observing viruses due to their nanometer-scale size.

Viral Structures and Their Functions

  • Capsid: Provides protection to viral nucleic acids, similar to a cell wall in bacteria and plants.

    • Consists of protein subunits assembled symmetrically.

  • Glycoproteins: Embedded in the viral envelope, which helps in targeting host cells through receptor interactions (e.g., COVID-19 glycoproteins interacting with respiratory tract cells).

Viral Entry Mechanisms

  • Direct penetration: A non-enveloped virus injects its nucleic acid directly into the host cell.

  • Membrane fusion: An enveloped virus integrates its lipid bilayer with the host cell membrane, facilitating entry.

Viral Genomic Considerations

Positive and Negative Sense RNA Viruses:

  • Positive sense RNA viruses: Can directly serve as mRNA for protein synthesis upon entering a host cell.

  • Negative sense RNA viruses: Require conversion to positive sense RNA before utilization for protein synthesis.

HIV Specific Mechanisms

  • HIV Life Cycle: HIV, a retrovirus, utilizes reverse transcriptase to convert its RNA into DNA within host cells.

  • The viral DNA integrates into host DNA, becoming dormant and evading immune detection.

  • Trigger conditions can lead to replication and production of new virions, resulting in clinical implications (e.g., AIDS).

Practical Implications in Virology

  • Due to the rapid mutation rates of viruses (like HIV and COVID-19), targeting viral-specific processes (like integrase) is essential for treatment strategies.