Unit 5 - Viruses

Viruses

Definition

  • Microscopic particles that infect cells

  • Infect both eukaryotes (complex cells) and prokaryotes (simple cells)

  • Ultra-microscopic size: 20 nm to 450+ nm in diameter.

  • Structure is compact and economical.

Properties of Viruses

  • Do not independently fulfill the characteristics of life.

  • Obligate intracellular parasites: active only once inside a host cell.

  • Virion: A fully formed virus capable of establishing infection in a host cell.

Basic Structure

  • Nucleic Acid Core: Contains either DNA or RNA, can be single or double-stranded.

  • Protein Coat (Capsid): Protects the nucleic acid.

General Structural Properties

  • Molecules on capsid surface are specific to host cells, allowing attachment.

  • Multiply by hijacking host cell’s genetic material, regulating the synthesis and assembly of new viruses.

Capsid

  • The protein shell surrounding the nucleic acid core.

  • Nucleocapsid: Combination of the capsid and nucleic acid.

  • May or may not have an outer envelope; if lacking, it is termed a naked virus.

Composition and Arrangement of Capsid

  • Made of identical protein subunits called capsomeres.

  • Self-assembly into a finished capsid; number of capsomeres varies among viruses.

  • Two arrangements: Helical and Icosahedral.

Viral Envelope

  • Acquired through the process of budding.

  • Contains proteins that form spikes on the outer surface, essential for attachment to the host cell.

Functions of Viral Envelopes/Capsids

  • Protects nucleic acids from enzymes and chemicals.

  • Introduces viral DNA/RNA into host cell.

  • Stimulates the immune system to produce antibodies, which protect the host against future infections.

Nucleic Acids

  • Contain either DNA or RNA; some can be single-stranded (ss) or double-stranded (ds).

  • RNA Types: Higher mutation rate, less stable, enables bypassing transcription.

  • DNA Types: Lower mutation rate, more stable, requires transcription for protein synthesis.

  • Example Viruses:

    • RNA: Coronaviruses, AIDS, retroviruses.

    • DNA: Adenoviruses, herpes viruses.

Viral Enzymes

  • Rely on host’s metabolic resources for replication due to lacking genes for metabolic enzymes.

  • Specific roles: Polymerases, replicases, and reverse transcriptase.

Bacteriophages

  • Viruses that infect bacterial cells; play a role in drug resistance.

  • Basic structure includes icosahedral head containing nucleic acid and a tail for attachment.

Viral Replication

  • Obligate intracellular parasites that cannot reproduce independently.

  • Viral replication may result in the death of the host cell.

Animal Virus Life Cycle

  1. Adsorption: Virus attaches to specific host receptors.

  2. Penetration: Virus enters host cell.

  3. Uncoating: Release of viral nucleic acid.

  4. Replication: Viral nucleic acids take over host’s machinery.

  5. Assembly: New virions are assembled.

  6. Release: Exit of new virions from the cell.

Adsorption and Penetration

  • Adsorption: Lock and key fit between receptors and viral proteins defines host range and infestability across species.

  • Penetration Mechanisms:

    • Direct fusion with the host cell membrane.

    • Endocytosis, where the cell engulfs the virus.

Assembly and Release

  • Assembly: Nucleic acid is inserted into the capsid to form the nucleocapsid.

  • Release: Two modes:

    • Enveloped viruses via budding or exocytosis.

    • Non-enveloped viruses upon host cell lysis.

Multiplication of Animal Viruses

  • Differentiated processes for naked and enveloped viruses they undertake various stages from adsorption to release using host cell machinery.

Viral Infections

Outcomes of Viral Infections

  • Abortive infections: Non-permissive host cells with no viral production.

  • Lytic (cytocidal) infections: Kill host cells to utilize their machinery.

  • Persistent infections: Chronic, non-lytic, with no immediate cell death.

  • Latent infections: Limited expression of viral genes during latency.

  • Transforming infections: Changes in host cell properties leading to malignancies.

Incubation Periods of Viral Infections

  • Influences how long it takes for symptoms to show after infection, varying by virus:

    • Influenza: 1-2 days

    • Common cold: 1-3 days

    • AIDS: Up to 10 years for symptoms.

Host Cell Damage

  • Morphological effects: Changes in cell shape, lysis, or altered membrane permeability.

  • Physiological effects: Altered cellular activities due to viral proteins.

  • Biochemical effects: Inhibition of macromolecule synthesis.

  • Genotoxic effects: Damage to host DNA, potential for mutations and oncogenesis.

Viral Identification and Cultivation

Purposes

  • Isolating and identifying viral specimens

  • Vaccine preparation and research on viral structures and cycles.

Methods of Culturing Viruses

  • Cannot grow without a living host.

  • Bacteriophages are easily cultured in bacterial hosts, while animal viruses require animal cells.

  • Techniques involve cell cultures and embryonated eggs.

Cell Cultures

  • Provide a sterile environment and nutritional growth requirements for viral multiplication and observation.

  • Continuous cell cultures allow for prolonged viral study and effect observation.

Embryonated Eggs

  • Used extensively in vaccine production; provides a living culture medium.

  • Signs of viral growth include embryo death and lesions.

Live Animal Inoculations

  • Some viruses can only be cultured in live animals, requiring ethical considerations for use.

Subviral Agents

  • Prions: Infectious proteins causing neurodegenerative diseases without nucleic acid.

  • Viroids: Short pieces of RNA lacking a protein coat, affecting plants.

  • Virusoids: Dependent on other viruses for replication.