GEN MICROBIOLOGY BIOL 2051
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This 9 page Class Notes was uploaded by Cassie Koepp on Tuesday October 13, 2015. The Class Notes belongs to BIOL 2051 at Louisiana State University taught by K. Sullivan in Fall. Since its upload, it has received 16 views. For similar materials see /class/222793/biol-2051-louisiana-state-university in Biological Sciences at Louisiana State University.
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Date Created: 10/13/15
Chapter 6 Virus Structure and Function General Properties of Viruses virions o Obligate intracellular parasites made of nucleic acid surrounded by a protein coat 0 Means viruses has to be inside host cell to reproduce 0 Nucleic acid of viruse can be DNA or RNA 0 Only replicate inside host 0 No nucleus organelles cytoplasm o Viralgenomes contain 0 information for taking over host cell 0 information for making viral proteins 0 Capsid genome replication proteins Virus Structure Capsids o Capsid protein coat surrounding nucleic acid 0 Capsid protein are arranged to give virion symmetry I icosahedral capsids o Roughly spherical o 20 equilateral triangles or faces 0 Many animal viruses I Filamentous capsids 0 Long tube of protien with genome inside 0 Tube made up of 100 s of identical protein subunits 0 Tube length reflects sixe of viral genome I Complex capsids o Mixture of icosahedral amp filamentous shapes 0 Many bacteriophage I Asymmetrical shapes o Tend to be larger viruses 0 Poxviruses Virus Structure Envelopes o Envelope lipid bilyaer around capsid of some viruses 0 Allows fusion of host cell or organelle membrane 0 Only occurs if host cell not covered by cell wall 0 Bacteriophages are NONenveloped o Envelope lipids come from host 0 Not enckise by viral genome 0 Proteins embedded in envelope may be encoded by virus 0 Coats viral capsid as virus leaves cell or organelle Virus Structure Genome 0 DNA or RNA 0 Single slandered ss or doublestranded ds 0 Linear or circular 0 Includes genes encoding viral proteins below are things that all have to have 0 Capsid o Envelope proteins if virus is enveloped 0 Any polymerase not found in host cell l Enzymes used to make RNA and protein Bacteriophage Life Cycles 1 Attachment to host cell receptor proteins 0 Bacterial cell receptors normally used for bacterial purposes 0 Not just there to be attachments for bacteria cells 0 Examples sugar uptake iron uptake conjugation 0 Virus takes advantage of host proteins 0 Host cell becomes a factory for making virus cells 2 injects genome through cell wall to cytoplasm of the cell 0 Force of injection is very strong bc has to be able to get through membrane 3 Replicate genome lytic vs lysogenic phage o Lytic cycle T4 0 Phage quickly replicates kills host cell 0 So host cell will lyse and release all the virus cells 0 Lysogenic cycle Lambda 0 Phage is quiescent 0 There but not doing anything not causing damage I May integrate into host cell genome I Replicates only when host genome divides o Everytime e coli replicates the virus copies too I Phage can reactivate to to become lytic kill host 9LysogenicTemperate phage 0 Enter a state called lysogeny where the viral genome integrates into host chromosome abd is replicated with host chromosome 0 Don t immediately make new phage particles 0 The virus is called a prophage when its genome exists as part of host genome 0 Bacterial host cells that harbor prophages are called lysogens o Eventually new phage are made lytic cycle occurs amp cells lyse 0 Induction of prophave lytic cycle host cell lysis l Switch from lyctic to other one 4 Use cell components to synthesize capsids 5 Assemble progeny phages 6 Exit from cell 0 Lysis 0 Makes protein to depolymerize peptidoglycan o Burts host cell to release progeny phage 0 Slow release 0 Filamentous phages can extrude individual progeny through cell envelope Quantification methods Plague assay 0 Used to count bacteria amp some animal viruses 0 When an infected cell lyses and releases phage those phage infect neighboring cells which causes them to lyse This cycle continues 0 This causes a a clearing in a lawn of bacteria called a plaque 0 Each plaque results from infection by a single virus particle Viral Restriction amp Modification by the Host 0 Bacteria synthesize enzymes as protection against viral invaders 0 Restriction endonucleases RE cleave DNA at specific sequences 0 Viruses modify their DNA to avoid digestion by the host s restriction endonucleases o Glucosylation o Methylation l Adding methyl groups to cyctosenses 0 proteins that inhibit restriction systems 0 Host cells have similar means of protecting their DNA from degrading by viral RE or by their own RE Overview of Animal Viruses 4 Types of infection by animal viruses 1 Lytic o Destruction of host cell just like bacteria phage casueits host to lyse this casues the cell to lyse 2 Persistent 0 New virions leave host by budding don t casue host to lyse all at once 0 Cell does not die but remains infected and will produce virions indefinetly becomes a factory for viruses for a very long time HIV 3 Latent 0 Virus is not actively replicating dormant 0 Symptoms appear only when virus emerges from latency example herpies 4 Transformation 0 Virus can change normal cell into cancer cell 0 Genetic changes that regulate growth cells will reproduce and grow uncontraolably and that s what casuees tumors 0 Benign or malignantsome cells from the tumor will break off and start growing tumors at other parts of body tumors Animal viruses 0 Can be single or double stranded DNA or RNA RNA viruses 0 Positivestrand RNA viruses 55 genomicone single stand RNA is used directly as mRNA for making proteins 0 Ex poliovirus coronaviruses amp rhinovirusescaue cold viruses 0 Negativestrand RNA viruses 55 0 Ex rhabdoviruses rabies Ebola virus measles influenza RSV o Doublestranded RNA viruses genomic RNA does not serve directly as MRNA but is transcribed into a complement that functions as mRNA 0 Reoviruses only animal virus with ds RNA 0 Rotavirus member of reovirus family most common cause of infant diarrhea DNA viruses Group o parvoviruses only group with singlestranded DNA 0 Herpesviruse group DS causes cold sores venereal disease chicken pox o Are able to remain latent in the body for years Becoming active during stressful conditions 0 Poxvirus group DS causes smallpox vaccinia cowpox amp some tumors Eukaryotic Virus Life Cycles 0 Attachment to host cell receptor 0 Attaches specifically to the receptor 0 Minto cell 0 ln39ects genome directlydirectly 0 Taken up via endocytosis o Brought into cell in an endosome o Menvelope to plasma membrane 0 Releases capsid into cytoplasm o Genome replication 0 DNA viruses must go to cell nucleus to use host polymerase 0 Or replicate in cytoplasm with viral polymerase 0 RNA viruses must encode a viral polymerase 0 Host cells cannot read RNA to make more RNA 0 To copy RNA into more RNA we don t have an enzyme that does that I All viruses make proteins with host ribosome o Translationprocess of making proteins occurs in cytoplasm I Assembly of new viruses 0 Capsid and genome Assembly may occur in cytoplasm or in nucleuscapsid proteins must move into nucleus Envelope proteins are inserted in host membrane plasma membrane or organelle membrane 0 Release of progeny viruses from host cell 0 Can Lysis ofcell 0 Similar to bacteria 0 This is one way they leave the cell 0 Or Budding 0 Another way to leave cell 0 Virus passes through membrane 0 Membrane lipids surround capsid to form envelope 0 All enveloped viruses bud from a membrane 0 Plasma membrane or organelle membrane Culturing Viruses on Plates 0 Viruses grown with host cells as a food source 0 Bacteria in culture 0 Animal cells in tissue culture 0 Host cells form confluent lawn total coverage of bacteria all bacteria 0 Viruses form plaques where hose cells killed 0 Plaque is a colony of viruses Retroviruses group Vl Retro means quotbackwardquot Cause HIV and some forms of cancer Single stranded RNA genome Enzyme reverse transcriptase copies RNA into DNA fairly unusal DNA integrated into host genome like a temperatelysogenic virus Viral DNA is transcribed into mRNA amp RNA for new viruses New viruses are assembled and released by budding Viral RNA AZT I reversetranscriptaseRT m ral DNA Host cell DNA Retrovirus DNA provirus Viroids and Prions Viroids Small nakes ss RNA viruses without capsids Mostly crop diseases Prions Protein only infectious agent BSE mad cow scrapie sheep CreutzfeldJakob kuru humans chronic wasting disease deer elk Normal protein takes on abnormal shape loses normal function Maybe ingest contaminated beef causes normal proteins to take on an abnormal shape and with proteins with their shape changes they can no longer work Viral Ecology Viruses present in all environments 0 Often at very high densitited 107 mL in ocean Viruses limit population host density 0 Hald of all bacteriua killed everyday by viruses every day myxovirus in rabbits Viruses increase host diversity 0 Strong selection for viruses resistant strains prevents dominance of one species People susceptible if exposed to new virus 0 West Nile Virus New virus now in most states 0 Smallpox virus in New World
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