nester13 - Important Point: Minimally, a virus is a...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Important Point: Minimally, a virus is a proteinaceous carrier of nucleic acid. Many viruses are more complicated than that, such as having a lipid envelope surrounding the protein capsid. What is a Phage? “The agents were called filterable viruses… Virus means ‘poison,’ a term that once had been applied to all infectious agents. With time, the adjective filterable was dropped and only the word virus was retained.” Bacteriophage were originally described as a macroscopic phenomenon that was slightly different from the “poisoning” of a plant or animal. Instead, what was observed was the destruction of a bacterial culture. People weren’t all that sure what a bacterium was so the destruction was seen more as an “eating” of the culture (by an otherwise unseen agent) rather than a poisoning. The filterable agent in this case was described as an “eater” of bacteria, or “Phage” from Greek: Bacteriophage = “Bacteria Eater.” “The agents were called filterable viruses… Virus means ‘poison,’ a term that once had been applied to all infectious agents. With time, the “The word was dropped and adjective filterable bacteriophage or only the phage that infect bacteria is both word virus was retained.” singular and plural when Bacteriophage were originally described as a referring to one type of virus. macroscopic phenomenon that was slightly The word phages is used when different from the “poisoning” of a plant or animal. different types of phages are Instead, what was referenced.” being observed was the destruction of a bacterial culture. People weren’t all that sure what a bacterium was so the destruction was seen more as an “eating” of the culture (by an otherwise unseen agent) rather than a poisoning. The filterable agent in this case was described as an “eater” of bacteria, or “Phage” from Greek: Bacteriophage = “Bacteria Eater.” Terms Describing Virions Virus Architecture What is a Phage? Chapter 13: Virus of Bacteria Virion is another name for virus particle. Virions are infectious meaning that they can deliver their nucleic acid to the cytoplasm of a susceptible cell (which for phages would be a bacterium). Capsid is the the protein coat that surrounds the nucleic acid and defines a virus as a virus. Capsids are made up of individual proteins called capsomers. The virion particle consists, minimally, of protein and nucleic acid which together is called a Nucleocapsid. Many particularly animal viruses have lipid bilayers surrounding the nucleocapsid; those viruses are described as Enveloped. Non-enveloped viruses are described as Naked. In enveloped viruses the envelope makes initial contact with cells and subsequent interaction with the cell surface is mediated by envelope proteins (proteins found in or on the envelope lipid bilayer). 1 Note spikes projecting from lipid envelopesurrounding capsid (which in turn surrounds the nucleic acid). Not-Complex Virions Virion Attachment Organs Virus Architecture Note spikes projecting from protein capsid surrounding nucleic acid. Some virions are isometric: they have a fully symmetrical capsids, almost spherical. Isometric virions attach to cells via Attachment Proteins, a.k.a., Spikes which are proteins that symmetrically project from their virions. Spikes project from capsids in Naked viruses. Spikes project from envelopes in Enveloped viruses. Some virions are helical with attachment proteins at end or ends. Most phages have tailed virions which sort of combine the morphology of isometric and helical virions, with the isometric part called a head and the helical part called a tail. Attachment is made at the end of the tail opposite the head, and often is mediated by thin “feelers” called tail fibers. Complex (Tailed) Phage Virion Note that this head actually is elongated top to bottom rather than isometric. “Big” “Small” ssDNA “Medium” “Even Smaller” Viral Genomes Virion Size` DNA dsDNA One way to distinguish different types of viruses (e.g., influenza virus from HIV) is in terms of the characteristics of their nucleic-acid genomes. Nucleic-Acid Virus Genome dsRNA Negative RNA ssRNA Positive ssRNA dsDNA ssRNA (the retroviruses) http://www.virology.net/Big_Virology/BVFamilyGenome.html 2 Viruses are Not Cells “Smaller” Some Important Phages Here complex means “tailed” “Bigger” Phages that are obligately lytic are called Virulent (vs. chronic or temperate). For phages this is called Lysogeny. Only Temperate phages are able to display lysogeny. For phages this is called Lysogenic Conversion. Chronic Infection Strategy This is the productiveinfection strategy followed by most phages including all tailed phages. An example of Lysogenic Conversion. This is a process called Extrusion. This is the strategy followed by filamentous phages. Viral Infection Strategies Latent Infection Strategy Lytic Infection Strategy Note various genome architectures. 3 Chronic Phage Life Cycle Temperate Phage Life Cycle(s) The prophage DNA can remain integrated indefinitely. G. Eliave Institute, Tbilisi, Georgia Virulent Phage Life Cycle Temperate Phage Life Cycle(s) Adsorption & Host Range Adsorption describes the virion’s attachment process. The virion contains proteins that recognize molecules found on the surface of cells (much as antibodies bind to antigens). Receptor Molecules (typically proteins) are molecules that cells make for various reasons (e.g., transport proteins) which viruses coopt for adsorption. Typically the Host Range of a virus is determined, at least in part, by the ability of the virus adsorption proteins to bind to host Receptor Molecules. Restriction-Modification systems can also limit phage host range (as well as biochemical incompatbilities). Through mutation (and other means) phages can overcome these host-range barriers, resulting, typically in a changed host range. Because of transduction, these changes in host range can allow gene exchange between even only distantly related bacteria. This is a process called Induction. 4 5 G. Eliave Institute, Tbilisi, Georgia G. Eliave Institute, Tbilisi, Georgia G. Eliave Institute, Tbilisi, Georgia G. Eliave Institute, Tbilisi, Georgia G. Eliave Institute, Tbilisi, Georgia G. Eliave Institute, Tbilisi, Georgia Link to Next Presentation 6 Virulent Phage Life Cycle 13 Chronic Phage Life Cycle 13 Temperate Phage Life Cycle(s) 13 ...
View Full Document

This note was uploaded on 06/03/2011 for the course MCB 205 taught by Professor Abedon during the Spring '11 term at Ohio State.

Ask a homework question - tutors are online