rav65819_ch27_523-538

rav65819_ch27_523-538 - ; 27 .036 mm .036 m chapter V...

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;;;;;;;; 27 .036 m m .036 m μ chapter
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Viruses introduction WE BEGIN OUR EXPLORATION of the diversity of life with viruses. Viruses are genetic elements enclosed in protein; they are not considered organisms since they lack many of the features associated with life, including cellular structure, and independent metabolism or replication. For this reason viral particles are not called viral cells, but virions, and they are generally not described as living or dead but as active or inactive. Because of their disease-producing potential, however, viruses are important biological entities. The virus particles pictured here produce influenza—flu for short. In the flu season of 1918–1919, an influenza pandemic killed approximately 20–50 million people worldwide, twice as many as were killed in combat during World War I. Other viruses cause such diseases as AIDS, SARS, and hemorrhagic fever, and some cause certain forms of cancer. For more than four decades, viral studies have been thoroughly intertwined with those of genetics and molecular biology. Classic studies using viruses that infect bacteria (known as bacteriophages ) have led to the discovery of restriction enzymes and the identification of nucleic acid, not protein as the hereditary material. Currently, viruses are one of the principal tools used to experimentally carry genes from one organism to another. Applications of this technology could include treating genetic illnesses and fighting cancer. 27.1 The Nature of Viruses Viruses are strands of nucleic acids encased in a protein coat Viral hosts include virtually every kind of organism Viruses replicate by taking over host machinery Most viruses come in two simple shapes Viral genomes exhibit great variation 27.2 Bacteriophages: Bacterial Viruses Bacterial viruses exhibit two reproductive cycles Bacteriophages can contribute genes to the host genome
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27.3 Human Immunode ciency Virus (HIV) f AIDS is caused by HIV HIV infection compromises the host immune system HIV infects key immune-system cells HIV treatment is a highly active eld of research f 27.4 Other Viral Diseases The u is caused by in uenza virus New viruses emerge by infecting new hosts Viruses can cause cancer concept outline 27.5 Prions and Viroids: Subviral Particles Prion replication was a heretical suggestion Evidence has accumulated that prions cause TSEs Viroids are infectious RNA with no protein coat 523 rav65819_ch27_523-538.indd 523 rav65819_ch27_523-538.indd 523 11/30/06 2:32:22 PM 11/30/06 2:32:22 PM 27.1 The Nature of Viruses All viruses have the same basic structure: a core of nucleic acid surrounded by protein. This structure lacks cytoplasm, and it is not a cell. Individual viruses contain only a single type of nucleic acid, either DNA or RNA. The DNA or RNA genome may be linear or circular, and single-stranded or double-stranded. RNA
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rav65819_ch27_523-538 - ; 27 .036 mm .036 m chapter V...

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