BIOL110: Module 3 – Lecture 4: Viruses

Viruses are infective nucleic acid-protein complexes that occur in all types of organisms

  • Viruses are biological entities defined by their inability to replicate outside of host cells.

  • They are classified as acellular microorganisms, distinguishing them from living prokaryotes (Archaea, Bacteria) and eukaryotes (Fungi, Protista, Animals, Plants).

The simplest viruses consist of a single or double stranded RNA or DNA molecule enclosed in a capsid made of one or more types of coat protein

  • Genome: Viruses contain genetic material in the form of either DNA or RNA.

  • Capsid: The genetic material is protected by a protein coat called a capsid, which can be rod-shaped, icosahedral, or a combination of both.

  • Envelope: Some viruses also possess an optional lipid envelope derived from the host cell membrane.

The viral genome codes for the coat protein, enzymes needed for viral nucleic acid replication, and other proteins essential for the infection process

  • Viruses are classified based on their genomic structure.

  • The genome carries specific instructions for structural and functional proteins, such as the S, E, M, and N proteins found in SARS-CoV-2.

The virus uses the host cell’s transcription and translation mechanisms to synthesise more viral genome and coat protein which self-assemble into virus particles

  • Infection Mechanism:

    • Entry: The virus binds to host cell receptors via its capsid proteins.

    • Synthesis: It usurps cellular machinery to carry out transcription and translation for viral components.

    • Self-Assembly: Demonstrated historically in 1955 with the Tobacco Mosaic Virus (TMV).

    • Release: New particles exit the cell, often resulting in host cell destruction.

Viruses are very diverse and many have complex structures and infection processes

  • Diversity is seen in the variety of capsid forms and genomic arrangements (DNA vs. RNA).

  • Rapid mutations, such as antigenic drift and shift in the Influenza virus, contribute to this high level of diversity.

Infection by viruses frequently causes disease

  • Common examples include Influenza and COVID-19.

  • Host Defense Mechanisms:

    • Mammals: Produce antibodies to block viral entry.

    • Plants and Insects: Utilize gene silencing to inhibit the production of viral RNA.

Viruses have some characteristics of living things, but they are not organisms. Their origins are a source of speculation

  • While they possess genetic material and evolve, they are acellular and lack independent metabolism, meaning they do not meet all traditional criteria for life.

Discovery of viruses

  • 1868: Identification of nucleic acid.

  • 1898: Discovery of the Tobacco Mosaic Virus (TMV).

  • 1939: First observation of a virus using an electron microscope.

  • 1955: Experimental demonstration of TMV self-assembly.

What they are

  • Viruses are biological entities consisting of genes (nucleic acids) and protein coats, existing strictly as obligate intracellular parasites.

How they work

  • They function by hijacking the genetic and protein-synthetic machinery of a host cell to produce offspring, terminating in the release of new virions.

Viruses as agents of disease

  • They are significant pathogens that trigger immune responses.

  • Vaccination efforts (e.g., Pfizer, AstraZeneca, Moderna) aim to prepare the immune system to recognize and fight these agents.

Diversity of viruses

  • Viral mutations can lead to zoonotic diseases, enabling viruses to cross species barriers and potentially cause global epidemics.