viruses

Viruses Overview

  • Definition: Infectious particles made of nucleic acid (RNA or DNA) surrounded by a protein coat (capsid).

  • Life Status: Not alive, exist between life and chemicals; some can crystallize.

  • Types of Genomes:

    • Double-stranded DNA

    • Single-stranded DNA

    • Single-stranded RNA

    • Double-stranded RNA

  • Capsid: Protein shell, may be rod-shaped (helical) or polyhedral (icosahedral).

  • Viral Envelope: Membranous cloak derived from host cell membranes.

  • Bacteriophage: Virus that infects bacteria.

Virus Replication

  • Obligate Intracellular Parasites: Can only replicate within a host cell.

  • Host Range: Limited number of species a virus can infect, determined by viral surface proteins and host cell receptors.

  • Infection Process:

    • Injection, endocytosis, or fusion of viral envelope with plasma membrane.

    • Virus reprograms host cell to replicate viral components.

  • Exit: Viruses exit by damaging or destroying the host cell.

Viral Life Cycles

  • Lytic Cycle: Leads to cell lysis and death of the host cell.

  • Virulent Phage: Only replicates via lytic cycle.

  • Lysogenic Cycle: Viral genome incorporated into bacterial chromosome (prophage), replicates with host, does not kill host.

  • Temperate Phage: Can replicate by either cycle.

Animal Viruses

  • Often have both an envelope and an RNA genome.

  • Glycoproteins bind to host cell receptors, facilitating entry.

  • Capsid disassembly releases viral genome for replication and protein synthesis.

  • Viruses exit the host via budding.

Retroviruses

  • RNA viruses that transcribe RNA into DNA and integrate it into host chromosomes.

  • Reverse Transcriptase: Enzyme used for DNA synthesis from RNA template.

Pathogenicity of Viruses and Prions

  • Symptoms Mechanisms:

    • Production of toxins.

    • Toxic molecular components damaging host cells.

    • Cell damage via hydrolytic enzymes.

  • Vaccines: Harmless variants that stimulate immune defenses.

  • Emerging Viruses: Recently noticeable due to increased transmissibility.

  • Epidemics: Widespread disease outbreaks enhanced by mutations and travel.

  • Plant Virus Transmission:

    • Horizontal (external source)

    • Vertical (parental inheritance)

  • Prion Definition: Misfolded proteins that increase in number by converting properly folded proteins.

  • Characteristics: Slow acting, long incubation (about 10 years), resist heat.