Micro Topic 6 - TOPIC 6: VIRUSES (chapter 13) 1. GENERAL...

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TOPIC 6: VIRUSES (chapter 13) 1. GENERAL CHARACTERISTICS OF VIRUSES a) Either very complex aggregations of nonliving chemicals or very simple living microbes. b) Have the following distinguishing features: a. Contain a single type of nucleic acid (DNA or RNA). b. Have a protein coat, sometimes enclosed by an envelope composed of lipids, proteins, and carbohydrates. The coat surrounds the nucleic acid. c. Multiply inside living cells, using the synthesizing machinery of the cell (obligatory intracellular parasites). d. Cause the synthesis of specialized structures that can transfer viral nucleic acid to other cells. A. HOST RANGE a) The host range is the spectrum of species, strains or cell types in which a virus can multiply. b) Most viruses infect only specific types of cells in one host species and they do not generally cross species barriers. c) The narrow host range of viruses and their ability to kill their host cells makes them potentially useful for treating disease: a. Phage therapy – using bacteriophages to kill bacterial infections. Problem: immune system b. Oncolytic viruses - tumor destroying viruses that infect and kill tumor cells or cause an immune response to tumor cells. Studies are underway. B. SIZE (Figure 13.1 pg 378) a) Viral size is determined by electron microscopy. b) Different viruses range from 20 to 1000 nm in length. 2. VIRAL STRUCTURE a) Virions (complete viruses) are composed of nucleic acid surrounded by a protein coat. b) They are classified by differences in their coat proteins. A. NUCLEIC ACID a) Viruses contain either DNA or RNA that may be single- or double-stranded, linear or circular, or divided into several separate molecules. 1
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B. CAPSID AND ENVELOPE a) The protein coat surrounding the nucleic acid of a virus is called the capsid. b) The capsid is composed of subunits called capsomeres. a. The capsomeres may be of one protein type or protein several types. b. Their arrangement is characteristic of a virus type. c) The capsid of some viruses is enclosed by an envelope: a. Consists of lipids, proteins and carbohydrates. b. May be covered with carbohydrate-protein complexes called spikes that help the virus attach to the host and can be used for identification. d) Viruses that do not have an envelope are non-enveloped viruses. e) When a virus infects a host, the host’s immune system produces antibodies in response to the virus coat proteins. Some viruses escape the immune system by changing their coat protein constantly e.g. influenza virus. C. GENERAL MORPHOLOGY (Figure 13.2 – 13.5 pg 379 – 381) a) Viruses may be classified by capsid architecture. a. Helical viruses (Ebola and rabies viruses) are long rods that may be flexible or rigid. Their capsids are hollow cylinders surrounding the nucleic acid. b.
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Micro Topic 6 - TOPIC 6: VIRUSES (chapter 13) 1. GENERAL...

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