IF

In-Depth Notes on Viruses

Overview of Viruses

  • Viruses are infectious particles consisting of a nucleic acid genome enclosed in a protein coat.
  • They cannot reproduce or metabolize outside of a host cell and are considered a form of "borrowed life."

Discovery of Viruses

  • Tobacco mosaic disease led to the hypothesis that small bacteria caused it.
  • In 1935, Wendell Stanley confirmed the existence of viruses by crystallizing Tobacco Mosaic Virus (TMV).

Structure of Viruses

  • Viruses are much simpler than prokaryotic cells and consist of a nucleic acid core surrounded by a protein coat (capsid).
  • Some viruses possess a membranous envelope derived from host cell membranes.

Viral Genomes

  • May consist of:
    • Double-stranded DNA or Single-stranded DNA
    • Double-stranded RNA or Single-stranded RNA
  • Can have between 3 to 2,000 genes.

Viral Replication

  • Viruses replicate only within host cells, acting as obligate intracellular parasites.
  • Viral replication involves the entry of the viral genome into the host and the production of viral proteins using the host's machinery.

Viral Replicative Cycles

  • Lytic Cycle: Results in the death of the host cell and release of new viruses. Phages involved are called virulent phages.
  • Lysogenic Cycle: Incorporates viral DNA into the host's genome without killing the host. Phages are termed temperate phages.
    • Prophage: Integrated viral DNA in the host.

Animal Viruses

  • Classically distinguished by the type of nucleic acid (RNA or DNA) and the presence of an envelope.
  • Many animal viruses acquire envelopes from host cell membranes.

Viral Envelopes

  • Viral glycoproteins on the envelope bind to host cell receptors for entry.
  • Retroviruses, like HIV, transcribe their RNA genome into DNA, integrating into the host DNA (provirus).

Evolution of Viruses

  • Viruses evolved from fragments of cellular nucleic acid and are categorized as mobile genetic elements (plasmids and transposons).

Pathogenic Viruses and Prions

  • Viruses and prions are significant pathogens impacting animals and plants.
  • Prions are misfolded proteins capable of inducing other proteins to misfold, leading to diseases such as Creutzfeldt–Jakob disease.

Viral Diseases

  • Viral infections can damage cells by releasing toxic components or hydrolytic enzymes.
  • Vaccines can prevent some viral illnesses but cannot treat existing infections; antiviral drugs can intervene in viral replication processes.

Emerging Viral Threats

  • Rapidly emerging viruses include HIV, Ebola, chikungunya, and Zika, often resulting from mutations or zoonosis.
  • Influenza epidemics often arise due to new viral strains and genetic reassortment in animal viruses.

Viral Diseases in Plants

  • Over 2,000 plant viral diseases; mainly transmitted through damaged cell walls or vertically from parent to offspring.

Prions as Infectious Agents

  • Prions cause degenerative brain diseases and can influence normal proteins to misfold, implicating them in conditions like Alzheimer’s and Parkinson’s disease.