26.1 Viruses are strands of nucleic acid encased within a protein coat.
The Nature of Viruses
• All viruses have the same basic structure: a nucleic acid core surrounded by a protein. (p. 532)
• Nearly all viruses form a capsid. (p. 532)
• Each virus can only replicate in a limited number of cell types, and the suitable cells are referred to as the host range. (p. 532)
• Viruses must enter cells and utilize cellular machinery to reproduce. (p. 532)
• The overall structure of a virus is either helical or isometric. (p. 532)
26.2 Bacterial viruses exhibit two sorts of reproductive cycles.
Bacteriophages
• Bacteriophages are viruses that infect bacteria. (p. 534)
• The lytic cycle refers to the reproductive cycle when a virus kills an infected host. Virulent viruses multiply within infected cells and eventually lyse the cells. (p. 534)
• Most bacteriophages do not immediately kill their hosts; these are referred to as prophages during lysogeny, the integration of a virus into a cellular genome. (p. 534)
Cell Transformation and Phage Conversion
• Transformation refers to the alteration of a cell's genome by the introduction of foreign DNA. (p. 535)
26.3 HIV is a complex animal virus.
AIDS
• In AIDS patients, the human immunodeficiency virus infects and kills CD4+ cells, eventually causing the patients to die of an infection that would normally be fought off by a healthy immune system. (p. 536)
• HIV can only enter cells that possess a cell-surface marker recognized by a glycoprotein on the HIV particle surface. (p. 536)
• The HIV infection cycle includes the following steps: attachment, entry into cells, replication, integration into the host cell's DNA, production of new viruses, and finally exit from the cells. (p. 536)
The Future of HIV Treatment
• The future of HIV treatment includes using combination drug therapy such as AZT analogs and protease inhibitors, using a defective HIV gene, using chemokines and CAF, and blocking or disabling receptors. (pp. 538-539)
26.4 Nonliving infectious agents are responsible for many human diseases.
Disease Viruses
• Viruses are known to cause many human diseases, including influenza, smallpox, infectious hepatitis, yellow fever, polio, AIDS, and SARS. (p. 540)
• Viruses have also been implicated in some cancers and leukemias. (p. 540)
• Emerging viruses originate in one organism and then pass to another. (p. 541)
Prions and Viroids
• Brain diseases known as transmissible spongiform encephalopathies are known to be caused by a protein called a prion (proteinaceous infectious particle). (p. 542)