Chapter Summary


  • Viruses consist of noncellular particles that infect a host cell and direct its expression apparatus to produce virus particles.
  • A virion consists of a capsid enclosing a nucleic acid genome. Some viruses are further enclosed by an envelope of membrane with embedded proteins.
  • All classes of organisms are infected by viruses. Usually the hosts are limited to a particular host range of closely related strains or species. ■ Viruses contain infective genomes that take over a cell, reprogramming its cell machinery to make progeny virus particles (virions). Some viral genomes consist of fewer than 10 genes; others have 100 or 200 genes and may represent degenerate cells.
  • Viroids that infect plants consist of RNA hairpins with no capsid.
  • Prions consist of infectious proteins that induce a cell’s native proteins to fold incorrectly and impair cell function.


  • The viral capsid is composed of repeated protein subunits—a structure that maximizes the structural capacity while minimizing the number of genes needed for construction.
  • The capsid packages the viral genome and delivers it into the host cell.
  • Icosahedral capsids have regular, icosahedral symmetry.
  • Filamentous (helical) capsids have uniform width, generating a flexible filamentous virion.
  • Enveloped viruses consist of a protein capsid and tegument proteins enclosed within phospholipid membrane derived from the host cell. The envelope includes virus-specific spike proteins.
  • Accessory proteins are contained within the capsid or as tegument components between the capsid and envelope.


  • Classification of viruses is based on genome composition, virion structure, and host range.
  • The Baltimore virus classification emphasizes the form of the genome (DNA or RNA, single- or double-stranded) and the route to generate messenger RNA.
  • Proteomic classification includes information from all viral proteins. Statistical analysis reveals common descent of viruses infecting a common host.


  • Host cell surface receptors mediate the attachment of bacteriophages to a cell and confer host specificity.
  • Lytic cycle. A bacteriophage injects its DNA into a host cell, where it utilizes host gene expression machinery to produce progeny virions.
  • Lysogeny. Some bacteriophages can insert their genome into that of the host cell, which then replicates the phage genome along with its own. A lysogenic bacterium can initiate a lytic cycle.
  • Slow release. Some bacteriophages use the host machinery to make progeny that bud from the cell slowly, slowing growth of the host without lysis.
  • CRISPR host defense. Host bacteria and archaea contain CRISPR sequences homologous to phage DNA. CRISPR RNA combines with Cas complex to defend against phage infection.


  • Host cell surface receptors mediate animal virus attachment to a cell and confer host specificity and tropism.
  • Animal DNA viruses either inject their genome or enter the host cell by endocytosis. The viral genome requires uncoating for gene expression.
  • RNA viruses use an RNA-dependent RNA polymerase to transcribe their messenger RNA.
  • Retroviruses use a reverse transcriptase to copy their genomic sequence into DNA for insertion in the host chromosome.
  • Plant viruses enter host cells by transmission through a wounded cell surface or an animal vector. Plant viruses travel to adjacent cells through plasmodesmata.
  • Pararetroviruses contain DNA genomes but generate an RNA intermediate that requires reverse transcription to DNA for progeny virions.


  • Culturing viruses requires growth in host cells. Bacteriophages may be cultured either in batch culture or as isolated plaques on a bacterial lawn.
  • Batch culture of viruses generates a step curve.
  • Animal viruses are cultured within animals or as plaques in a tissue culture.
  • Fluorescent-focus assays reveal foci of virusinfected cells.
  • Oncogenic viruses are cultured as foci of cancer-transformed host cells.


  • Environmental change results in new emerging viral pathogens.
  • Host population density is limited by virus infection, and host genetic diversity is increased.
  • Virus-to-bacteria ratios range from 10–100 for marine and aquatic environments, and 10–1,000 for soil.
  • Marine viruses are the major consumers of phytoplankton. Viral activity substantially impacts the global carbon balance.