Chapter 23: Viral Replication and Taxonomy

Viruses are presented as obligate intracellular parasites that cannot reproduce independently and must hijack host cell machinery to complete their life cycles. The replication process follows a defined sequence beginning with viral attachment to host cell surface receptors, followed by penetration across the cell membrane, uncoating to release genetic material, genome replication, synthesis of viral proteins, assembly of new viral particles called virions, and finally cell exit through either lytic rupture or membrane budding. Understanding viral structure is essential for comprehending replication strategies and includes three critical components: the genetic material itself which may be DNA or RNA in single or double-stranded configurations and may be segmented across multiple particles, the protein shell or capsid which can adopt icosahedral, helical, or complex geometric arrangements, and the lipid envelope derived from host membranes that surrounds some viruses and contains viral glycoproteins. The chapter contrasts DNA and RNA virus replication pathways, noting that DNA viruses typically replicate within the nucleus using host polymerases while RNA viruses most commonly replicate in the cytoplasm and frequently encode their own RNA-dependent RNA polymerase enzyme. A critical distinction emerges between positive-sense RNA which functions immediately as messenger RNA and negative-sense RNA which must first be transcribed into positive-sense before translation. Retroviruses represent a specialized category employing reverse transcriptase to convert their RNA genome back into DNA for chromosomal integration. The chapter systematically categorizes major viral families including DNA viruses such as Herpesviridae, Adenoviridae, Papillomaviridae, Parvoviridae, Poxviridae, and Hepadnaviridae alongside RNA viruses including Picornaviridae, Orthomyxoviridae, Paramyxoviridae, Rhabdoviridae, Retroviridae, Flaviviridae, and Coronaviridae, providing practical organizational tools for clinical recognition and board examination success.