Viruses

Structure and Classification

• Viruses are obligate intracellular parasites, not cells: one nucleic acid type (DNA or RNA, never both) inside a protein capsid, sometimes wrapped in a host-derived lipid envelope. Enveloped viruses are fragile (soap, alcohol, drying); naked viruses persist in the environment.
• Baltimore logic: dsDNA viruses (herpes) use host polymerases; (+)ssRNA genomes (coronaviruses) are mRNA-ready and translate immediately; (-)ssRNA viruses (influenza, Ebola) must package their own RNA-dependent RNA polymerase because host cells cannot copy RNA from RNA; retroviruses (HIV) carry reverse transcriptase; hepatitis B uses a reverse-transcribing dsDNA scheme.

Replication Cycles

• Lytic cycle: attach, inject, replicate, assemble, lyse, release.
• Lysogenic cycle: phage DNA integrates as a prophage and replicates silently with the host until stress triggers a lytic switch.
• Retroviruses: $\text{ssRNA} \xrightarrow{\text{reverse transcriptase}} \text{dsDNA} \xrightarrow{\text{integrase}} \text{provirus}$ Reverse transcriptase lacks proofreading, so HIV mutates rapidly — the rationale for combination therapy.