Overview
This lecture covers the fundamental structure, classification, and reproductive mechanisms of viruses, explaining why viruses are not considered living organisms.
What Is a Virus?
- Viruses are non-living particles much simpler and smaller than unicellular organisms like bacteria.
- Viruses do not perform metabolism or reproduce independently.
- They exist in a gray area between simple molecules and living organisms.
- A typical virus consists of genetic material encased in a protein shell with no membrane or organelles.
Virus Structure
- Viral genetic material can be DNA or RNA, single- or double-stranded, linear or circular.
- The protein shell enclosing the genetic material is the capsid, made of capsomeres.
- Virus shapes include rod-shaped (helical), icosahedral, enveloped with spikes, and complex forms like bacteriophages.
How Viruses Infect and Reproduce
- Viruses infect host cells by recognizing specific cell surface receptors.
- Infection can occur by direct genetic injection (e.g., bacteriophages) or endocytosis (enveloped viruses).
- Viruses lack ribosomes and depend on host machinery for gene expression and replication.
- New viral particles assemble inside the cell and exit, sometimes destroying the host cell.
Viral Replication Cycles
- The lytic cycle ends with host cell lysis, releasing many new viruses.
- The lysogenic cycle integrates viral DNA (prophage) into the host genome, allowing cell division before possibly switching to lytic mode.
- Enveloped viruses may enter/exit cells without destroying them via endocytosis or exocytosis.
Special Types of Viruses and Infectious Agents
- Retroviruses use reverse transcriptase to transcribe RNA into DNA.
- Viroids are infectious naked circular RNA molecules affecting plants.
- Prions are infectious misfolded proteins causing brain diseases without any genetic material.
Viral Evolution and Host Interaction
- Bacteria evolve resistance through mutations in surface receptors; viruses evolve by altering surface proteins.
- Both viruses and their hosts are in a constant state of evolutionary change.
Key Terms & Definitions
- Virus — Non-living particle with genetic material in a protein coat, requiring a host for replication.
- Capsid — Protein shell that encases viral genetic material.
- Capsomere — Subunit protein composing the capsid.
- Lytic cycle — Viral replication that results in host cell destruction.
- Lysogenic cycle — Viral DNA incorporates into host genome and can later become active.
- Prophage — Viral DNA integrated into a bacterial genome.
- Retrovirus — Virus using reverse transcriptase to copy RNA into DNA.
- Viroid — Infectious naked circular RNA, affecting plants.
- Prion — Infectious misfolded protein particle causing disease.
Action Items / Next Steps
- Prepare for the next lecture on the biological structure of the simplest organisms.