Overview
This lecture covers the biology, classification, and replication of viruses, viroids, and prions, including their structures, life cycles, and the diseases they cause.
Virus Structure & Characteristics
- Viruses are too small for light microscopes and cannot be cultured outside a host cell.
- Considered non-living as they are inert outside a host and cannot reproduce independently.
- Obligatory intracellular parasites: require living host cells for multiplication.
- Contain either DNA or RNA, never both; lack cell membrane, ribosomes, and ATP-generating enzymes.
- Structure: nucleic acid core (DNA or RNA) surrounded by a protein capsid, sometimes with a lipid envelope.
- Host range is determined by specific virus attachment sites on host cells.
Classification & Types of Viruses
- Classified by nucleic acid (DNA or RNA, single/double-stranded, linear/circular) and capsid structure.
- Major capsid types: helical (e.g., rabies), polyhedral (e.g., polio), enveloped (e.g., influenza, herpes), complex (e.g., bacteriophage).
- Genus names end in -virus; family names end in -viridae.
- Viral species are defined by genetic information and host range, usually given descriptive common names.
Viral Replication Cycles
- Viruses must be grown in living cells; bacteriophages create plaques in bacterial lawns.
- Lytic cycle: attachment, penetration, biosynthesis, maturation, release; ends with host cell death.
- Lysogenic cycle: viral DNA integrates into host genome as a prophage, remaining latent until triggered.
- Animal virus replication also involves attachment, entry (endocytosis or fusion), uncoating, biosynthesis, maturation, and release (rupture or budding).
DNA and RNA Viruses
- DNA viruses replicate in the host nucleus; examples: Adenoviridae, Poxviridae, Herpesviridae, Papovaviridae, Hepadnaviridae.
- RNA viruses replicate in the cytoplasm using RNA-dependent RNA polymerase; single-stranded (+) sense can act as mRNA; (-) sense must synthesize a (+) sense copy first.
- Retroviruses (e.g., HIV) use reverse transcriptase to produce DNA from RNA and integrate into the host genome.
Viruses and Disease
- Some viruses cause cancer by integrating oncogenes into host DNA (oncogenic viruses).
- Viral infections can be acute, latent (dormant, reactivating), or persistent (chronic).
Viroids & Prions
- Viroids: infectious RNA without a protein coat, affecting plants.
- Prions: infectious misfolded proteins causing neurological diseases by inducing normal proteins to misfold (e.g., mad cow disease, Creutzfeldt-Jakob disease).
- Prion diseases are not destroyed by radiation and resist breakdown by normal enzymes.
Key Terms & Definitions
- Capsid — protein coat surrounding viral nucleic acid.
- Envelope — lipid, protein, and carbohydrate coating outside the capsid in some viruses.
- Bacteriophage — virus that infects bacteria.
- Prophage — integrated phage DNA in bacterial genome.
- Oncogene — gene that can transform a normal cell into a cancer cell.
- Viroid — infectious naked RNA particle.
- Prion — infectious protein causing brain diseases (spongiform encephalopathies).
Action Items / Next Steps
- Review classification and replication cycles of viruses, especially differences between DNA, RNA, and retroviruses.
- Prepare for potential bonus questions on prions and types of viruses (e.g., cause of COVID-19).
- Study examples of diseases caused by each virus type and their mechanisms.