Chapter 11: Viral Genomics and Diversity

Viral genomes display remarkable variation in structure and composition, consisting of either DNA or RNA that may be single stranded or double stranded and organized as linear, circular, or segmented molecules. The Baltimore classification system categorizes viruses into seven groups based on how their genomes generate messenger RNA, providing a framework for understanding viral molecular biology and replication strategies. Viral taxonomy and evolutionary relationships are determined using a polyphasic approach that integrates phenotypic traits, genome sequence data, host range, structural morphology, and replication mechanisms under the guidance of the International Committee on Taxonomy of Viruses. The chapter explores major groups of DNA viruses, including single stranded bacteriophages such as phiX174 and filamentous phages like M13 that replicate through double stranded replicative forms and rolling circle replication. Double stranded bacteriophages such as T4, T7, and lambda illustrate complex replication programs involving specialized DNA polymerases, genome packaging mechanisms, and regulatory pathways that determine lytic or lysogenic development. Archaeal viruses are highlighted for their unusual morphologies and genomic structures, demonstrating the diversity of viral evolution across different domains of life. Animal DNA viruses are also discussed, including poxviruses that replicate in the cytoplasm, adenoviruses that use terminal proteins for DNA replication, and tumor viruses such as polyomaviruses and herpesviruses that can establish persistent infections and sometimes contribute to cancer development. The chapter then examines RNA viruses, which require RNA dependent RNA polymerases to replicate their genomes and produce viral messenger RNA. Positive strand RNA viruses such as poliovirus and coronaviruses use their genome directly as mRNA and often produce polyproteins that are later cleaved into functional viral proteins, whereas negative strand RNA viruses such as rabies and influenza must first synthesize complementary mRNA using viral replicases carried within the virion. Double stranded RNA viruses such as reoviruses replicate within protective nucleocapsid structures to shield their genomes from host defenses. Finally, the chapter introduces subviral agents including viroids, which are small circular RNA molecules that infect plants and replicate using host enzymes, and prions, infectious proteins capable of propagating misfolded protein structures without nucleic acids, illustrating the diverse molecular forms that infectious agents can take.