BacteriologyChapter9.pptVIP

Chapter 9 Virology Extracellular vs. Intracellular Viruses Extracellular viruses are inactive, do not carry on metabolism, are not undergoing replication, etc. Extracellular viruses = virus particles or virions Infection = when viruses gain entry into a host cell. Viruses are obligate intracellular parasites: require the machinery of a host cell in order to undergo replication, etc. Viral Genomes Genetic material = DNA or RNA (only retroviruses contain both at a certain stage of their reproductive cycle) Genetic material may be ss or ds, ex. ssDNA, dsDNA, ssRNA, dsRNA. Viruses are often classified according to what type of genetic material they contain. All viruses obey the Central Dogma: DNA ? RNA ? protein (although, retroviruses carry out an extra step called reverse transcription) Viral Hosts Viruses are also classified based on what type of host they infect. Animals, plants, and bacteria can all be infected by viruses. Bacterial viruses are called bacteriophages (or phages for short = “to eat”) There is a formal system of viral taxonomy featuring orders, families, genera, and species. Virus family names have the suffix -viridae. Viral Structure Viruses are composed of: (1) genome, (2) protein coat (= capsid, coat, shell) (3) optional envelope (lipid membrane). Capsid subunits = capsomers, which undergo self-assembly to provide a protective coat surrounding the genome. The finished genome surrounded by capsid = nucleocapsid. Viruses vary widely in size and shape. They range from 0.02 ?m (20 nm) - 0.3 ?m (300 nm) in size. Virus Symmetry Viruses are geometrical and highly symmetrical in shape. 2 types of symmetry: 1. Helical - rod-shaped viruses 2. Icosahedral - spherical viruses. Enveloped Viruses The virus envelope consists of a lipid bilayer with proteins embedded in it. The lipids