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18.1 Studying Viruses and Prokaryotes

18.1 Studying Viruses and Prokaryotes

18.1 Studying Viruses and Prokaryotes

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FIGURE 18.4 Viral ShapesSome viruses attach to host cells by these spikes. The spikes are such anobvious trait of some viruses that they can be used for identification.<strong>Viruses</strong> can only reproduce after they have infected host cells. <strong>Viruses</strong> aresimply packaged sets of genes that move from one host cell to another. Unlikebacteria <strong>and</strong> other living parasites, a virus has no structures to maintain—nomembranes or organelles needing ATP, oxygen, or glucose. All it carries intothe cell is what it needs to reproduce—its genes.The structure <strong>and</strong> shape of viruses play an important role in how theywork. Each type of virus can infect only certain hosts. A virus identifies itshost by fitting its surface proteins to receptor molecules on the surface of thehost cell, like a key fitting a lock. Some viruses are able to infect several species,while other viruses can infect only a single species. Common viral shapes areshown in FIGURE 18.4.The different proteins that make up a viral capsid give viruses a variety of shapes.ENVELOPEDHELICALPOLYHEDRALEnveloped viruses, such as thisinfluenza virus, often have spikes.The envelope is shown in orange.(colored TEM; magnification 255,000)capsidnucleic acidlipidenvelopesurfaceproteinsSome viruses have a long, narrow,coiled shape called a helix. Therabies virus is an example of ahelical virus that also has an envelope.(colored TEM; magnification65,000)surface proteinscapsidnucleic acidlipidenvelopePolyhedral viruses are many-sided,like the one shown here thatcauses foot-<strong>and</strong>-mouth disease inanimals. (computer illustration)surfaceproteinscapsidnucleic acidCompare <strong>and</strong> Contrast What are the similarities <strong>and</strong> differences between the threetypes of viruses shown above?548 Unit 6: Classification <strong>and</strong> Diversity

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