TAILIEUCHUNG - Lecture Biology: Chapter 19 - Niel Campbell, Jane Reece

Chapter 19 introduce to Viruses. The main contents of this chapter include all of the following: Explain how capsids and envelopes are formed, distinguish between the lytic and lysogenic reproductive cycles, explain why viruses are obligate intracellular parasites, describe the reproductive cycle of an HIV retrovirus, describe three processes that lead to the emergence of new diseases, describe viroids and prions. | Chapter 19 Viruses Overview: A Borrowed Life Viruses called bacteriophages can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli. Viruses lead “a kind of borrowed life” between life-forms and chemicals. The origins of molecular biology lie in early studies of viruses that infect bacteria. A virus consists of a nucleic acid surrounded by a protein coat. Tobacco Mosaic Virus Causes Disease RESULTS 1 2 3 Extracted sap from tobacco plant with tobacco mosaic disease Passed sap through a porcelain filter known to trap bacteria Rubbed filtered sap on healthy tobacco plants 4 Healthy plants became infected Figure What causes tobacco mosaic disease? Structure of Viruses : Viruses are not cells. Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein coat and often a membranous envelope. Viral genomes may consist of either: Double- or single-stranded DNA or Double- or single-stranded RNA Capsids and Envelopes A capsid is the protein shell that encloses the viral genome. Capsids are built from protein subunits called capsomeres. Some viruses have membranous envelopes that help them infect hosts. Viral envelopes surround the capsids of viruses and enable the viruses to evade detection by the host likely because viral envelopes are derived from the host cell’s membrane. Viruses / Structures RNA Capsomere Capsomere of capsid DNA Glycoprotein 18 250 nm 70–90 nm (diameter) Glycoproteins 80–200 nm (diameter) 80 225 nm Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 50 nm 50 nm 50 nm 20 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4 Figure Viral structure Bacteriophages, also called phages, are viruses that infect bacteria. Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell. Each virus has a host range, a limited number of host cells that it can infect. Transcription and manufacture of capsid .

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