Chapter 5: Viruses and Their Multiplication

Viruses are noncellular genetic entities composed of nucleic acid genomes surrounded by a protective protein capsid, and in some cases a lipid envelope derived from the host cell membrane. Because they lack metabolic machinery, viruses function as obligate intracellular parasites that depend on host cells for energy, biosynthetic processes, and protein synthesis. The chapter describes virion structure, including capsids built from capsomeres, nucleocapsids formed by the genome and capsid, and structural symmetry patterns such as helical, icosahedral, and complex head–tail morphologies. Methods used to culture and quantify viruses are introduced, including plaque assays that measure viral titer in plaque-forming units. The viral replication cycle is explained through sequential stages of attachment to host receptors, penetration of viral genetic material into the host cell, synthesis of viral nucleic acids and proteins using host machinery, assembly of new virions, and release through mechanisms such as cell lysis or budding. Bacteriophage T4 serves as a model system illustrating the lytic replication pathway, including genome injection, temporal gene expression, virion assembly, and host cell destruction. The chapter also discusses temperate bacteriophages that establish lysogeny by integrating their genome into the host chromosome as a prophage, allowing viral DNA to replicate with the host until induction triggers entry into the lytic cycle. Viral infection strategies in eukaryotic cells are explored, including entry through membrane fusion or endocytosis, genome replication in the nucleus or cytoplasm, and outcomes such as lytic infection, persistent infection, latency, or cellular transformation. Plant virus infection is also examined, emphasizing the role of wounds and biological vectors in viral transmission and the movement of viruses between plant cells through plasmodesmata.