Overview
This lecture covers the discovery, evolution, morphology, and classification systems of viruses, emphasizing their diversity and the Baltimore classification scheme.
Discovery and Detection of Viruses
- Viruses were discovered using filters that removed bacteria, revealing filterable infectious agents.
- Most viruses are 20-250 nanometers; some, like amoebae viruses, can be up to 1000 nm.
- Electron microscopes, not light microscopes, are needed to visualize most viruses.
- Virions (single virus particles) have been studied using scanning and transmission electron microscopy.
Evolution of Viruses
- Virus origins are uncertain due to lack of fossils; information comes from genetic and biochemical studies.
- Three main hypotheses: regressive (from free-living cells), progressive (from mobile genetic elements), and virus-first (pre-cellular origins).
- Viruses likely have multiple evolutionary origins and continue evolving with their hosts.
Viral Structure and Morphology
- Virions lack cellular structures and consist of nucleic acid core, protein capsid, and sometimes an envelope.
- Capsomeres are protein subunits forming the capsid.
- Four main capsid types: helical, icosahedral, enveloped, and head-and-tail.
- Glycoproteins on the virus surface enable cell attachment via host cell receptors.
- Example structures: T4 bacteriophage (complex), adenovirus (spiked capsomeres), influenza virus (enveloped with glycoproteins).
Types of Viral Nucleic Acid
- Viral genomes can be DNA or RNA, single- or double-stranded, linear or circular, and segmented or non-segmented.
- RNA viruses mutate faster than DNA viruses due to error-prone RNA polymerases.
- Examples: DNA viruses (chickenpox, herpes), RNA viruses (influenza, HIV).
Virus Classification Methods
- Viruses were initially classified by structure, nucleic acid type, and envelope presence.
- No universal genomic sequence exists to classify viruses like cellular organisms.
- Past systems used genome and capsid structure for grouping.
Baltimore Classification System
- Groups viruses based on their genome and how they produce mRNA:
- I: dsDNA (e.g., Herpes simplex)
- II: ssDNA (e.g., Canine parvovirus)
- III: dsRNA (e.g., Rotavirus)
- IV: ssRNA (+) (e.g., Picornavirus)
- V: ssRNA (-) (e.g., Rabies virus)
- VI: ssRNA with reverse transcriptase (e.g., HIV)
- VII: dsDNA with reverse transcriptase (e.g., Hepatitis B)
Key Terms & Definitions
- Virion — a single virus particle.
- Capsid — protein shell enclosing a virus’s nucleic acid.
- Capsomere — subunit of a capsid.
- Glycoprotein — viral surface molecule for host cell attachment.
- Enveloped virus — virus surrounded by a lipid membrane from the host cell.
- Baltimore classification — system grouping viruses by genome type and mRNA synthesis route.
Action Items / Next Steps
- Review virus morphology diagrams and the Baltimore classification table.
- Study key examples of each viral group and their replication strategies.
- Prepare for quiz on viral structures and classification systems.