Ch13Notes - Chapter 13-Viruses, Viroids, and Prions I....

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Chapter 13—Viruses, Viroids, and Prions I. General Characteristics of Viruses. a. Are inert when not within a host cell. b. Require a living host cell to multiply. i. They are therefore considered to be obligate intracellular parasites. c. Viruses contain DNA or RNA, but not both. d. No ATP generating mechanism. e. Viruses have a protein coat surrounding the nucleic acid. i. Some viruses are also enclosed by an outer lipid envelope. f. Little or no metabolic activity. g. Viruses multiply inside living cells by using the host cell’s own enzymes, nucleic acids, amino acids, ATP, etc. i. Most drugs that would interfere with viral multiplication would have the same effect on the host cell. h. Host Range: i. The range of host cells the virus can infect. ii. The host range of a virus is determined by specific host attachment sites and cellular factors necessary for viral multiplication. 1. Attachment sites include things such as cell walls, fimbriae, flagella, and plasma membrane proteins. iii. Most viruses infect only specific types of cells in one host species. 1. Bacterial viruses are called bacteriophages or phages. i. Viral Size: Fig. 13.1. i. Range from 20 to 1000 nm in length. 1. Remember, prokaryotes range in size from 1 µ m to 10 µ m, while eukaryotes range in size from 10 µ m to 500 µ m. II. Viral Structure. a. Virion: A single mature, complete, infectious viral particle. b. Nucleic Acid. i. DNA or RNA is the primary genetic material in a given species of virus. ii. Can be either a single strand nucleic acid or double strand nucleic acid: 1. Single strand DNA (ssDNA). 2. Double strand DNA (dsDNA). 3. Single strand RNA (ssRNA). 4. Double strand RNA (dsRNA). iii. The nucleic acid can be linear or circular. 1. In some viruses, the nucleic acid is in several separate segments. a. Influenzavirus. b. Bird flu and genetic reassortment. c. Capsid and Envelope. Fig. 13.2. i. Capsid: Protein coat surrounding the nucleic acid of a virus. 1. Composed of subunits called capsomeres. a. Proteins composing the capsomeres may all be the same, or they may be composed of several different proteins. ii. Envelope: Lipid layer external to the capsid. Fig. 13.3. 1. Also contains proteins and carbohydrates. a. Some animal viruses become coated with the host cell’s plasma membrane when released. 2. May or may not be covered with “spikes.” a. Glycoproteins (antigens) that project from the surface of the envelope.
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i. In some viruses, these glycoproteins serve as attachment points for binding to host cells. 3. Viruses that have an envelope are said to be “enveloped viruses.” iii. Nonenveloped Viruses: 1. Do not have an envelope. iv.
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Ch13Notes - Chapter 13-Viruses, Viroids, and Prions I....

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