Viruses

• Tiniest viruses are only 20nm in diameter-smaller than a ribosome.
• Viruses have no metabolism, they cannot reproduce without the host cell.
• The virus is made up of nucleic acid (DNA or RNA) surrounded by protein coat called capsid
• Viral genomes can be made of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the type of virus.
• A virus is called a DNA virus or an RNA virus based on the kind of nucleic acid that makes up its genome.
• Viral genomes are usually organized as a single linear or circular molecule of nucleic acid, although the genomes of some viruses consist of multiple molecules of nucleic acid.
• The smallest viruses known have only three genes in their genome, while the largest have several hundred to 2,000.
• For comparison, bacterial genomes contain about 200 to a few thousand genes.
• Capsids are built from a large number of proteins subunits called capsomeres.
• Some viruses have accessory structures that help them infer their hosts.
• A membranous envelope surrounds the capsids of influenza viruses and many other viruses found in animals.
• These viral envelopes, which are derived from the membranes of the host cell, contain host cell phospholipids and membrane proteins.
• viral envelopes also contain proteins and glycoproteins of viral origin.
• Glycoproteins are proteins with carbohydrates covalently attached.
• Some viruses carry a few viral enzyme molecules within their capsid.
• Many of the most complex capsids are found among the viruses that infect bacteria, called bacteriophages or phages.
• Viruses lack metabolic enzymes and equipment for making proteins such as ribosomes.
• Viruses are obligate intracellular parasites; they can replicate only within a host cell.
• In isolation, viruses are merely packaged sets of genes in transit from one host cell to another.
• Each particular virus can infect cells of only a limited number of host species, which is referred to as the host range of the virus.
• The host specificity of viruses results from the evolution of recognition systems by the virus.
• Viruses usually identify host cells by a "lock-and-key" fit between viral surface proteins and specific receptor molecules on the outside of cells.
• According to one model, receptor molecules originally carried out functions that benefited the host cell but were co-opted later by viruses as portals of entry.
• Some viruses have broad host ranges, for example, West Nile virus and equine encephalitis virus are distinctly different viruses that can each infect mosquitoes, birds, horses, and humans.
• Other viruses have host ranges so narrow that they infect only a single species, for example, Measles virus can infect only humans.
• Viral infection of multicellular eukaryotes is usually limited to particular tissues, for example, human cold viruses infect only the cells lining the upper respiratory tract, and HIV seen in Figure 19.1 binds to receptors present only on certain types of immune cells.
• The mechanism of genome entry depends on the type of virus and the type of host Cell.
• A viral infection begins when a virus binds to a host cell, and the viral genome makes its way inside.
• The mechanism of a genome entry, depends on the type of virus, and the type of host cell.
• T-even phages use their elaborate tail apparatus to inject DNA into a bacterium.
• Other viruses are taken up by endocytosis or, in the case of enveloped viruses, by fusion of the viral envelope with the host's plasma membrane.
• Once the viral genome is inside, the proteins it encodes can commandeer the host, reprogramming the cell to copy the viral genome and manufacture viral proteins.
• The host provides the nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins.
• Many DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA.
• In contrast, to replicate their genomes, RNA viruses use virally encoded RNA polymerases that can use RNA as a template.
• Uninfected cells generally make no enzymes for carrying out this process.
• Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses after they are produced.
• Researchers can separate the RNA and capsomeres of TMV and then reassemble complete viruses simply by mixing the components together under the right conditions.
• The simplest type of viral replicative cycle ends with the exit of hundreds or thousands of viruses from the infected host cell, a process that often damages or destroys the cell.
• Cellular damage and death, as well as the body's responses to this destruction, cause many of the symptoms associated with viral infections.