13.1 General Characteristics of Viruses

• Viruses were originally distinguished from other infectious agents because they are especially small (filterable) and because they are obligatory intracellular parasites—that is, they absolutely require living host cells in order to multiply.

• Viruses are entities that:

  • Contain a single type of nucleic acid, either DNA or RNA.

  • Contain a protein coat (sometimes itself enclosed by an envelope of lipids, proteins, and carbohydrates) that surrounds the nucleic acid.

  • Multiply inside living cells by using the synthesizing machinery of the cell.

  • Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells.

• The host range of a virus is the spectrum of host cells the virus can infect.

• Viruses that infect bacteria are called bacteriophages, or phages.

13.2 Viral Structure

• The nucleic acid of a virus is protected by a protein coat called the capsid.

  • Each capsid is composed of protein subunits called capsomeres.

  • In some viruses, the capsid is covered by an envelope, which usually consists of some combination of lipids, proteins, and carbohydrates.

• Depending on the virus, envelopes may or may not be covered by spikes, which are carbohydrate-protein complexes that project from the surface of the envelope.

• Viruses whose capsids aren’t covered by an envelope are known as nonenveloped viruses.

• Some viruses, particularly bacterial viruses, have complicated structures and are called complex viruses.

• One example of a complex virus is a bacteriophage.

13.3 Taxonomy of Viruses

• A viral species is a group of viruses sharing the same genetic information and ecological niche (host range).

13.4 Isolation, Cultivation, and Identification of Viruses

• Plaques are visible against a lawn of bacterial growth on the surface of the agar.

• Therefore, the concentrations of viral suspensions measured by the number of plaques are usually expressed in terms of plaque-forming units (PFU).

• Certain cell lines, called diploid cell lines, developed from human embryos can be maintained for about 100 generations and are widely used for culturing viruses that require a human host.

• When viruses are routinely grown in a laboratory, continuous cell lines are used.

13.5 Viral Multiplication

• The multiplication of viruses can be demonstrated with a one-step growth curve.

• The lytic cycle ends with the lysis and death of the host cell, whereas the host cell remains alive in the lysogenic cycle.

  • The second result of lysogeny is phage conversion; that is, the host cell may exhibit new properties.

  • The third result of lysogeny is that it makes specialized transduction possible.

• Many viruses enter into eukaryotic cells by receptor-mediated endocytosis.

• Uncoating is the separation of the viral nucleic acid from its protein coat.

• These viruses carry reverse transcriptase, which uses the viral RNA as a template to produce complementary double-stranded DNA.

• The viral DNA is then integrated into a host cell chromosome as a provirus.

13.6 Viruses and Cancer

• These cancer-causing alterations to cellular DNA affect parts of the genome called oncogenes.

  • Oncogenes were first identified in cancer-causing viruses and were thought to be a part of the normal viral genome.

• Viruses capable of inducing tumors in animals are called oncogenic viruses, or oncoviruses.

• Tumor cells undergo transformation; that is, they acquire properties that are distinct from the properties of uninfected cells or from infected cells that don’t form tumors.

• After being transformed by viruses, many tumor cells contain a virus-specific antigen on their cell surface, called tumor-specific transplantation antigen (TSTA), and transformed cells tend to be irregularly shaped, compared to normal cells.

13.7 Latent Viral Infections

• The classic example of such a latent infection is the infection of the skin by Simplexvirus, which produces cold sores.

13.8 Persistent Viral Infections

• A persistent viral infection (or chronic viral infection) occurs gradually over a long period.

13.9 Plant Viruses and Viroids

• Some plant diseases are caused by viroids, short pieces of naked RNA, only 300 to 400 nucleotides long, with no protein coat.

  • Some viroids, called virusoids, are enclosed in a protein coat.

13.10 Prions

• Prusiner coined the name prion for proteinaceous infectious particles.