by 123 chapter 19 - Chapter 19: Viruses Viruses are not...

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Chapter 19: Viruses Viruses are not alive, but exist in a shady area between life-forms and chemicals The discovery of viruses started with the search for what caused tobacco mosaic disease o After bacteria-removing filtrations of sap, the sap still produced mosaic disease o Infectious agent in sap could reproduce o Infectious agent could not be cultivated on nutrient media in test tubes or petri dishes Viruses are smaller than ribosomes Viruses are infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Viruses genomes may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, according to the nuclei acid that makes up its genome A virus is called either DNA or RNA virus The genome is usually organized as a single linear or circular molecule of nucleic acid. The smallest viruses have only 4 genes Capsid - protein shell enclosing the viral genome, built from capsomeres Viral envelopes - a membranous envelope around viruses that is derived from the membranes of the host cell, contain host cell phospholipids and membrane proteins, along with proteins and glycoproteins of viral origin o Tobacco mosaic virus has a helical capsid with the overall shape of a rigid rod o Adenoviruses have an icosahedral capsid with a glycoprotein spike at the vertex o Influenza viruses have an outer envelope studded with glycoprotein spikes. The genome consists of eight different RNA molecules, each wrapped in a helical capsid o Bacteriophage T4 , like other “T-even” phages, has a complex capsid consisting of an icosahedral head and a tail apparatus. Viruses are obligate intracellular parasites - they can only reproduce within a host cell Each type of virus can infect cells of only a limited variety of hosts, called the host range of the virus o Viruses identify cells by a “lock-and-key” fit between viral surface proteins and specific molecules on the outside of each cell o Measles can infect only humans, but West Nile, Rabies, and Bird Flu have a wide host range o Viral infections of multi-cellular eukaryotes is usually limited to particular tissues Human cold viruses affect only the cells lining the upper respiratory tract
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by 123 chapter 19 - Chapter 19: Viruses Viruses are not...

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