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Viral Structure and Activities Summary

• Viruses have an outer layer consisting of a phospholipid bilayer and viral proteins, and a nucleocapsid containing nucleic acid and protein in enveloped viruses.
• Virulent (lytic) infection destroys the host, redirecting host cell metabolism to support multiplication and virion assembly, while lysogenic infection alters the host cell genetically by integrating viral genome into the host genome.
• Viruses can be classified based on the nature of their genetic material (DNA or RNA), single-stranded or double-stranded, and the hosts they infect (bacterial, animal, plant, etc.).
• Some viruses exert beneficial effects, such as increasing drought tolerance in plants or decreasing HIV replication and infectivity.
• Virion structure includes capsomeres, with some viruses having a single capsid protein due to the small size of viral genomes, and can be assembled spontaneously or with host cell folding assistance.
• Viruses exhibit two primary shapes, rod and spherical, with helical and icosahedral symmetry respectively, and some viruses have complex structures like head-plus tail bacteriophages.
• Enveloped viruses have a nucleocapsid surrounded by a lipoprotein membrane and use outer surface proteins for attachment and infection, while some have specific enzymes like lysozyme, neuraminidases, and nucleic acid polymerases inside virions.
• Culturing and detecting bacteriophages can be done using plaque assay, which measures the titer (number of infectious virions per volume of fluid).
• Animal cell culture and viral plaques are similar to plaque assay of bacteriophages, and the efficiency of infection is usually much lower than 100% due to defective virions or inappropriate conditions for infectivity.
• The replication cycle of prokaryotic viruses involves attachment, penetration, synthesis of viral nucleic acid and proteins, assembly of capsids, and release of new virions from the host cell.
• The growth curve of viruses includes an eclipse phase, maturation, latent period, and release, with burst size indicating the number of virions released.
• Attachment and entry of bacteriophage T4 to the host cell require complementary receptors on the host cell surface, with receptors carrying out normal host functions and varying depending on the phage.