In-Depth Notes on Viruses and Viral Diseases

Overview of Viruses
  • Viruses are infectious particles consisting of nucleic acids (DNA or RNA) encased in a protein coat, often with a lipid envelope derived from host cells.
  • Distinct from cells, viruses can replicate only within a host cell, marking them as obligate intracellular parasites.
Structure of Viruses

  • Viral Composition:

    • A virus comprises nucleic acid (either DNA or RNA) and a protein coat called a capsid. The capsid is made of protein subunits known as capsomeres.
    • Enveloped Viruses:
    • Many viruses have a membranous envelope, which is comprised of host cell lipids. These enveloped viruses use their glycoproteins to attach to host cell receptors for entry (e.g., SARS-CoV-2 and ACE2).

  • Types of Viral Genomes:

    • Viral genomes may consist of:
    • Double- or single-stranded DNA (dsDNA or ssDNA).
    • Double- or single-stranded RNA (dsRNA or ssRNA).

  • Capsid Structure:

    • Capsids may exhibit various structures, and some, like bacteriophages, have complex shapes, including tails for attaching to host bacteria.
Viral Replicative Cycles

  • General Mechanism:

    1. Attachment: Virus binds to a host cell.
    2. Entry: Viral genome enters the host cell (via penetration or endocytosis).
    3. Replication: Host cell machinery is hijacked to manufacture viral components.
    4. Assembly: Newly formed viral particles self-assemble and exit the cell, often destroying it in the process.

  • Lytic vs. Lysogenic Cycle in Phages:

    • Lytic Cycle: Causes cell death through lysis after producing new phages.
    • Lysogenic Cycle: Integrates phage DNA into the host's chromosome as a prophage; can activate to enter the lytic cycle later.
Viruses in Human Health

  • Viral Diseases:

    • Viruses can disrupt cellular functions or cause cell death by triggering immune responses or producing toxic proteins.
    • Common examples include HIV, the flu, and COVID-19.

  • Prevention and Treatment:

    • Vaccines: Stimulate an immune response without causing disease.
    • Antiviral Medications: Work by targeting various viral lifecycle stages; do not cure infections but mitigate effects.
Evolution and Emerging Viruses

  • Emerging Viruses:

    • New or previously unrecognized viruses that cause diseases (e.g., Ebola, Zika, SARS-CoV-2).
    • Spread can occur through mutation, environmental changes or interspecies transmission.

  • Causes of Viral Emergence:

    • High mutation rates of RNA viruses; zoonotic spillovers from animal populations; localized outbreaks can evolve into widespread pandemics.
Viral Diseases in Plants
  • Plant viruses (over 2,000 types known) share structures and replication methods similar to animal viruses, with most being RNA-based.
  • Transmission:
    • Horizontal transmission through external sources (e.g., insects) or vertical transmission from infected parent plants.
Prions: Infectious Proteins
  • Prions are misfolded proteins that cause neurodegenerative diseases in animals and are extremely stable and resistant to standard treatment.
  • Their mechanism involves inducing normal proteins to misfold, leading to cellular dysfunction.
  • Examples: Things like Mad Cow Disease and potential involvement in human diseases like Alzheimer's and Parkinson's.