Chapter 26: Viruses – Classification, Life Cycles & Infection

Chapter 26 details the diverse and complex reproductive strategies employed by viruses, beginning with their classification via the hierarchical International Committee on Taxonomy of Viruses (ICTV) system and the functional Baltimore system, which organizes viruses into seven groups based on their genome type and mRNA synthesis pathway. Double-stranded DNA (dsDNA) viruses, such as the lytic T4 bacteriophage (known for its chemically modified DNA and genome concatemers) and the temperate Lambda phage (a critical model for gene regulation controlling the switch between lytic and lysogenic cycles using regulatory proteins like cI, cII, and Cro), often leverage host polymerases for replication and transcription. Eukaryotic dsDNA viruses like herpesviruses cause both acute productive infections and lifelong latent infections, replicating in the host nucleus and requiring intricate processes for transport and secondary envelopment. Other notable dsDNA viruses include Nucleocytoplasmic Large DNA Viruses (NCLD or megaviruses), which are exceptionally large and encode much of their own replication and translation machinery. Single-stranded DNA (ssDNA) viruses must synthesize a double-stranded replicative form (RF) intermediate to serve as a template; examples include ϕX174 and parvoviruses, which utilize host enzymes and specialized methods like rolling-hairpin replication. RNA viruses face the challenge of requiring RNA-dependent RNA polymerase (RdRp), an enzyme not found in host cells, which functions both as a transcriptase (making mRNA) and a replicase (copying the genome). Double-stranded RNA (dsRNA) viruses (like rotaviruses and ϕ6) use this RdRp within the core particle. Plus-strand RNA (+ssRNA) viruses, such as poliovirus, use their genome directly as mRNA upon entry, often translating a single polyprotein that is subsequently cleaved, while employing specific mechanisms like the Internal Ribosome Entry Site (IRES) to bypass host requirements. Minus-strand RNA (-ssRNA) viruses, exemplified by the segmented influenza virus, must package the RdRp within the virion; influenza further utilizes a technique called "cap snatching" to steal primers from host mRNAs. Finally, reverse transcribing viruses utilize Reverse Transcriptase (RT) to generate DNA from an RNA template; retroviruses (like HIV) integrate their new dsDNA copy (the provirus) into the host chromosome, while reverse transcribing DNA viruses (like Hepatitis B Virus) use RT to replicate their gapped DNA genome from an intermediate RNA pregenome.