Virology Flashcards

Viruses

What is a Virus?

  • A very small infectious, obligate intracellular parasite.
  • Requires a host cell to replicate its genome and synthesize its component parts.
  • Progeny virions (new virus particles) formed in host cells transmit the virus to the next host cell.

Properties of Life vs. Viruses

  • Properties of Life: Cellular Respiration, Reproduction, Metabolism, Heredity, Responsiveness, Growth and development.
  • Viruses: Utilize the host cell to carry out their processes, replicate to produce new virions, and spread to a new host.

Host Cell Resources Used by Viruses

  • Viruses rely on host cells for:
    • Nucleotides (for nucleic acid production).
    • Amino acids (for translation of proteins).
    • Ribosomes (for translation of proteins).
    • ATP (for energy).
    • Golgi apparatus (for protein processing).
    • Endoplasmic reticulum (for protein processing).

Classification of Viruses

  • Viruses are classified based on:
    • Nucleic acid: Double-stranded (ds) or single-stranded (ss) DNA or RNA.
    • Capsid symmetry: Helical, Icosahedral, or Complex.
    • Presence or absence of an envelope: Naked or Enveloped.
    • Genome characteristics.

Viral Components

  • A virus contains:
    • Nucleic acid (DNA or RNA).
    • Protein coat (capsid) surrounding the nucleic acid.
    • Sometimes an envelope (outer lipid layer).
    • Sometimes a capsule (additional protective layer).

Virus Structure

  • Naked Virus:
    • Composed of nucleic acid surrounded by a capsid.
    • Capsid is made up of capsomers (protein subunits).
  • Enveloped Virus:
    • Contains nucleic acid, capsid, and an envelope.
    • The envelope is derived from the host cell membrane and contains viral glycoproteins.

Virus Nucleocapsid

  • Highly symmetrical.
  • Composed of one or more protein subunits (capsid proteins).
  • Capsomeres (capsid proteins) are arranged in a specific manner to form the capsid.
  • The capsid is formed by self-assembly of the capsomeres.

Examples of Viruses

  • Viruses infect various hosts, including:
    • Bacteria (Bacteriophages).
    • Plants.
    • Animals.

Bacteriophages

  • Infect and replicate within bacteria.
  • Highly virulent for bacteria.
  • Possess a complex structure.
  • Classified by shape and nucleic acid type.

Bacteriophage Structure

  • Composed of:
    • Head.
    • Collar.
    • Tail.
    • Tail pins.
    • Tail fibers.

Bacteriophage Life Cycle

  • A five-step process:
    1. Attachment: Virus binds to specific receptors on the host cell surface.
    2. Penetration/Entry (injection): Viral genome enters the host cell.
    3. Synthesis of nucleic acid and protein: Viral components are produced using host cell machinery.
    4. Assembly and packaging: New virions are assembled.
    5. Release (lysis): New virions are released from the host cell.
1) Attachment
  • All viruses have attachment proteins on their surface.
  • These proteins bind to specific receptor proteins on the host cell surface.
  • Virus receptors are normal proteins found on the outside of the host cell.
  • The host immune response can target these attachment proteins.
2) Penetration or Entry
  • Bacteriophages inject their DNA into the host cell.
3) Synthesis of Nucleic Acid Genome and Proteins
  • Example: Bacteriophages T4 and Lambda infecting E. coli.
    • T4 is virulent (causes host cell death).
    • Lambda is temperate and has two life cycles: lytic and lysogenic.
    • The choice between lytic and lysogenic cycles depends on complex genetic events.
  • Lytic Cycle: Infected bacteria become production factories for lambda bacteriophages and are lysed to release new phages.
  • Lysogenic Cycle: Bacteriophage genome is incorporated into the bacterial host chromosome via genetic recombination at a specific site.
    • Prophage: The term for bacteriophage DNA when it is inserted into the bacterial host chromosome.
    • The prophage is replicated along with the host DNA during cell division.
4) Assembly and Packaging
  • Viral components are assembled into virions.
5) Release (Lysis)
  • The host cell lyses (breaks open), releasing new virions.

Plant Viruses

  • Example: Tobacco Mosaic Virus (TMV).
    • Helical symmetry.
    • Composed of 2130 identical capsomeres.
    • Helical nucleocapsid containing viral RNA.

Virus Entry into Plant Cells

  • Viruses enter plant cells through various mechanisms.
  • Once inside, they replicate and create "virus factories".

DNA and RNA Viruses

  • DNA Viruses:
    • Non-enveloped: Parvovirus (ssDNA), Papovavirus (dsDNA), Adenovirus (dsDNA), Iridovirus (dsDNA).
    • Enveloped: Hepadnavirus (partially dsDNA), Poxvirus (dsDNA), Herpesvirus (dsDNA).
  • RNA Viruses:
    • Non-enveloped: Picornavirus (ssRNA), Reovirus (dsRNA).
    • Enveloped: Togavirus (ssRNA), Rhabdovirus (ssRNA), Orthomyxovirus (ssRNA), Bunyavirus (ssRNA), Coronavirus (ssRNA), Paramyxovirus (ssRNA), Arenavirus (ssRNA), Retrovirus (ssRNA).

Non-enveloped Viruses

  • Example: Poliovirus (Picornavirus).
  • Typically have icosahedral capsids containing more than 60 capsid proteins.
  • Foot and mouth disease virus is another example.

Enveloped Viruses

  • The envelope is derived from the cell, Golgi, or nuclear membrane.
  • Enveloped viruses bud or burst from the cell.
  • Examples: Measles virus, influenza virus.

Virus Glycoproteins

  • Matrix protein links the virus nucleocapsid to the membrane.
  • Virus glycoproteins are inserted in the membrane and form spikes on the outside of the virus.
  • Serve as the main antigens for the immune system.

Virus Entry Mechanisms

  • Direct Penetration: Virus directly penetrates the cell membrane.
  • Membrane Fusion: Viral envelope fuses with the cell membrane.
  • Endocytosis: The host cell cytoplasmic membrane engulfs the virus.

Enveloped Virus Life Cycle

  • Envelope (lipid bilayer with glycoprotein spikes) fuses with the cell membrane.
  • Synthesis of nucleic acid genome and viral proteins.
  • The endoplasmic reticulum of cell makes viral glycoproteins.
  • Site of virus assembly varies (Golgi complex, nuclear or cell membrane).
  • Progeny viruses bud from the cell (similar to exocytosis).
  • Cell membrane remains intact - does not kill host cell.

Example: Human Immunodeficiency Virus (HIV).

  • Single-stranded enveloped RNA virus.
  • Belongs to the Retroviridae family.
  • RNA genome is converted (reverse transcribed) into double-stranded DNA by the virus's reverse transcriptase.
  • Viral DNA is imported into the nucleus and integrated into the host DNA.
  • Virus latency allows virus to avoid detection by the immune system.
  • Alternatively, the virus produces new virus particles that are released from the cell.

HIV Components

  • Surface Envelope Protein.
  • Transmembrane Envelope Protein.
  • Lipid Membrane Bilayer.
  • Core Shell Protein.
  • Core Protein.
  • Reverse Transcriptase.

HIV Entering Cell

  1. Viral RNA enters the host cell.
  2. Reverse transcriptase creates an RNA-DNA hybrid.
  3. Chromosomal DNA is targeted.
  4. Provirus DNA is integrated.
  5. Viral proteins are created.
  6. New HIV RNA is produced, leading to the creation of new HIV viruses.

Virus Culture

  • Viruses are obligate intracellular parasites (need cells to live).
  • Grown in cell culture to observe the effect of the virus on cells (cytopathic effect).
  • Different types of cells are used.
  • Sometimes eggs can also be used (e.g., for influenza virus).

Types of Cell Culture

  • Primary Cell Culture: Obtained from live animal tissue; has a limited lifespan.
  • Diploid Cell Lines: Homogeneous cells from embryos; divide about 100 times and then die.
  • Continuous Cell Lines: Immortal cell line (do not stop growing), usually from cancerous cells.

Cytopathic Effect

  • Observable effects of a virus on cells under a microscope:
    • Transformation: Tumor formation.
    • Lysis: Cell death.
    • Persistent infection of the cell.
    • Latent infection of the cell (can become lytic).

Quantification of Viruses (Virus Titre)

  • End Point Dilution Assay.
  • Plaque Assay: Can also be used to isolate a single virus.
    • Virus is inoculated on a monolayer of cells or bacteria.
    • An overlayer of agar and medium is added.
    • The virus can only spread from cell to cell.
    • Holes or 'plaques' of dead cells form in the monolayer.
    • Modifications can be used if the virus does not form plaques.