EBIO 3400 1st Edition Lecture 22Outline of Last Lecture I. AlphaproteobacteriaII. GammaproteobacteriaIII. BetaproteobacteriaOutline of Current Lecture II. Parasitic Particles: Viruses, Viroids, and PrionsIII. Tobacco Mosaic VirusIV. T4 BacteriophageV. Evolution of virusesVI. Animal Viruses Current LectureI. Parasitic Particles: Viruses, Viroids, and PrionsA. Virus: non-cellular particle capable of infecting a host cell where it reproducesB. Virion: complete virus particle (extracellular• Consists of DNA or RNA enclosed in a coat of protein• DNA or RNA but not both• unable to reproduce outside of living cells• obligate intracellular parasites• bacteriophage = virus that infects bacteriaC. Extracellular State: Called virion. - Protein coat (capsid) surrounding nucleic acid- Some have phospholipid envelope -Outermost layer provides protection and recognition sites for host cellsD. Intracellular State (initially, before replication):Capsid removed-Virus exists as nucleic acidThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.E. Enveloped virus Many animal viruses..Spike proteins are also often “attachment proteins” that attach to specific sites host cell. Envelope derived from host plasma membraneII. Tobacco Mosaic Virus- Genome = 1000 bp- Genes code for capsomer and one other unknown protein- 60 identical capsomers: 60 protein units - Only infects plant cells already infected with the tobacco mosaic virus – seems to commandeer machinery from the tobacco mosaic virus – uses TMV as host!- Only see it when cell already infected with mosaic virus, TMV encodes of more proteins for the virus to reproduce A. Always are naked viruses (no membrane)Important for: 1. Genetic transfer (transduction) 2. Control of Bacteria in Nature About 108 phage per ml of sea water! 3.Transduction can cause bacteria to become pathogens (e.g. Diphtheria, Cholera) B. Host ranges of phage:- Phage are specific to the species they infect - they attach to specific receptors on the outer layers of the bacterium - e.g. some phage of E. coli attach specifically to theproteins of the flagellum… - Phage for a particular bacterium also have DNA methylation patterns like their host and thus avoid having their DNA cleaved by restriction enzymes when the DNA enters the cell iii. T4 Bacteriophage- dsDNA, lytic life cycle, infects E. coli. Generalized transduction results from the lytic cycle. Chromosome is digested into small pieces - some of which canend up being packaged in virus particles - Viral plaque formation - zones of clearing are due to lytic viral cyclesA. Lambda bacteriophage- dsDNA, infects E. coli, - capable of a lysogenic life cycle (a temperate phage)-Lysogenic conversion: phenotype of host cell changed by presence of temperate phage- Prophage = the lysogenic phage incorporated into the bacterial chromosomeWhat is the advantage of being lysogenic (= latent)?1) Host dormancy = long term survival2) Reproduced along with host chromosome3) Lytic cycle kills hosts - thus host may go extinct if virus is too efficient at killing hostsIV. Evolution of viruses- Reductive Evolution: (virus evolved from independent or intracellular bacteria)vs.Escaped Gene Theory: (virus evolved from ‘escaped’ DNA or RNA that gained independence from cellular control)- If virus kills host quickly, spreading to new cells will be more difficult- Narrow host range (viruses are usually only able to infect very specific type of cells because of dependence on host proteins)- Coordination of viral gene expression and assembly inside host cellSpace limitations (genome size restricted by capsid size)V. Animal Viruses A. The International Committee on Taxonomy of Viruses (ICTV) has devised a classification system, based on several criteria:a. Genome compositionb. Capsid symmetryc. Enveloped. Size of the virione. Host rangeB. Classification of animal viruses ssDNAdsDNAssRNAdsRNAvirion morphology, enveloped vs. nonenvelopedhost rangeWhy is it difficult to determine the phylogenetic relationships between viruses?A) viral genomes are too smallB) many viruses cannot be isolated in cultureC) there are no conserved genes across all virusesD) viruses accumulate mutations very
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