Chapter 11: Viruses- part 1
Chapter 11: Viruses- part 1 2420
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This 2 page Class Notes was uploaded by Siân L'Roy on Monday August 22, 2016. The Class Notes belongs to 2420 at Tarrant County College District taught by Mark Pulse in Summer 2016. Since its upload, it has received 5 views. For similar materials see Microbiology in Biology at Tarrant County College District.
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Date Created: 08/22/16
Viruses Part I Each virus is identified as a virus particle or virion (single virus) Virus particles consists of DNA surrounded by a protein coat known as a capsid o A capsid is made up of identical proteins known as capsomers o If there is only a capsid, then it is identified as a ‘naked’ virus particle o Some particles have an outer lipid covering, identified as enveloped viruses (mostly animal viruses) o Capsid associated projections are identified as attachment spikes; envelope associated projections are matrix proteins Three different particle (capsid) shapes exist: 1. Isometriccapsid: are made of flat, equilateral triangles 2. Helicalcapsid: are rodshaped 3. Complexisometric: head is attached to a long tail projection and only occurs in bacterial population The viral(phage) infection type depends on the replication mechanism of the virus after infection; two general types of infections occur: o Productive infection: multiple in host cell and escape by lysing(lytic) or leaking(extrude) mechanisms; they are usually quick and rapid o Temperate infection: phage DNA replicates as the host DNA replicates; infection may not be evident(latent), but can change to lytic stage(phage induction); they are usually slow, provirus and release by budding Phage that reproduce through a productive lytic infection of the host cell is identified as a virulent phage T4 cell are a doublestranded DNA phage that infects E. coli and reproduces through a productive lytic infection T4 replication in E. coli occurs in six outlined steps: o Attachment (Adsorption): Protein fibers on the tail attach to specific cell wall receptors on host cell o Penetration: Lysozyme degrades a portion of the cell wall and dsDNA is injected into the host cell (phage; DNA(viral) inside; capsid outside) o Transcription: Transcription of host DNA stops, phage DNA is transcribed producing phageencoded proteins; early proteins include a nuclease that degrades Viruses Part I the host DNA and capsidassociated proteins (removing the competition; breaking up the host DNA) o Replication: phage DNA is replicated and more proteins are synthesized using the host’s machinery o Assembly(maturation): 1. Phage head formed 2. dsDNA is incorporated 3. Assembled tail is attached to head 4. Tail spikes and fibers are added to the end of the tail o Release: phageencoded lysozyme is produced internally, which digests the cell wall and results in cellular lysis Phage lambda reproduces through a temperate/productive infection in E. coli o Phage DNA integrates into the host (chromosomal) DNA through sitespecific recombination o Integrated DNA is vertically passed onto progeny; a phageencoded repressor protein helps maintain integrated DNA o Phage DNA is excised and the phage enters into the productive lytic stage o Phage capsid structures are produces, excised DNA is packaged, and particles are assembled o Particles are released by cellular lysis Filamentous phage reproduce through a temperate infection of F+ E. coli o Filamentous phage attach(adsorb) to the host cell at the F+ pilus and the phage DNA is injected o Phage DNA replicated in the carrier (host) cell o As DNA is extruded (leaked) from the carrier cell, cytoplasmic associated capsomer proteins assemble to form protein coat o Phage particles are released without lysing the carrier cell