Viruses 🦠

• Viruses vs. Cells

Viruses lack organelles, cytoplasm, and membranes.

They consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid), sometimes with a lipid envelope.

Prokaryotic and eukaryotic cells have membranes, cytoplasm, ribosomes, and metabolic machinery.

• Properties of Life in Viruses

Viruses can evolve and reproduce, but only inside a host. They do not independently metabolize, grow, or maintain homeostasis.

• DNA Viruses

Contain DNA as genetic material; replicate using host DNA polymerase (e.g., herpesvirus).

Low mutation rate; only require one vaccination

• RNA Viruses

Contain RNA as genetic material; often replicate using RNA-dependent RNA polymerase (e.g., influenza virus).

High mutation rate (RNA polymeres cannot proofread DNA); may require yearly vaccinations

• Lytic Cycle

Virus immediately replicates inside host, causing cell lysis and death (e.g., bacteriophage T4).

• Lysogenic Cycle

Viral DNA integrates into host genome, replicates silently until triggered to enter lytic cycle (e.g., bacteriophage λ).

• Vaccination: One-Time

Stable DNA viruses (e.g., measles) require one vaccination because their antigens rarely change.

• Vaccination: Annual

Rapidly mutating RNA viruses (e.g., influenza) require annual vaccines due to frequent antigenic shifts/drifts.