Micro Chapter 13 Notes PDF

Preview

Summary

MICRO...

Description

Chapter 13 

Tobacco mosaic disease 1892 o D. M. Iwanowsky determined caused by filterable virus o Too small to be seen by microscope o Later discovered that virus destroyed bacteria



Virus: means poison



Virus o DNA containing or RNA containing viruses o In one virus is cannot have both; DNA & RNA o No metabolism, replication, motility o Infectious agents, but not alive o Can be classified by the type of cell they affect (pro or eu)  Bacteriophages affect prokaryotes o May provide alternative to antibiotics



Characteristics of Viruses o Small size  ~10 nm,~10 genes o Largest  ~500 nm



Virion (viral particle) o Protein coat is capsid—protects nucleic acids  Carries required enzymes and is composed of identical subunits called, capsomers

o Capsid plus nucleic acid= nucleocapsid o Enveloped viruses have a lipid bilayer envelope o Matrix protein between nucleocapsid and envelope o Naked viruses lack envelope and are more resistant to disinfectants 

Viral genome either DNA or RNA, never both o Useful for classification o Genome linear or circular o Double or single stranded  Affects replication strategy



Viruses have protein components for attachment o Phages have tail like fibers o Animal viruses have spikes o Allow virion to attach to specific receptor sites



Viruses have 3 different shapes o Icosahedral o Helical o Complex



Virus families end in suffix –viridae o Some indicate appearance o Others named for geographic area



Genus ends in –virus o Ex: enterovirus



Species names often name of disease o Ex: poliovirus causes poliomyelitis

2



Viruses commonly referred to by species name





Bacteriophages o 3 general types  Lytic phages  Temperate phages  Filamentous phages



Lytic Phage Infections o Lytic or virulent phages exit host o Cell is lysed  Productive infection: new particles formed o T4 phages (dsDNA) as model; process takes ~30 mins o 5 step process  attachment: phage exploits receptors  genome entry: t4 lysozyme degrades cell wall and injects genome  synthesis of protein and genome  assembly: maturation  release: lysozyme digests cell wall, releases phage



3

Temperate phage Infections

o Option of lytic infection or incorporation of DNA into host cell genome  Lysogenic infection  Infected cell is lysogen o Lambda phage: linear chromosome, single strand o Resulting circular molecule either directs lytic infection or integrates into E. Coli chromosome o Phage enzyme integrase inserts DNA at specific site  Integrated phage DNA termed prophage  Replicates with host chromosome  Results in lytic infection o DNA excised from chromosome only once per 10000 divisions o If damaged, SOS repair system activates protease which destroys repressor and allows prophage to be excised, enter lytic cycle  Called phage induction: allows phage to escape damaged host o Lysogen immune to superinfection and lysogenic conversion  Change in phenotype of lysogen from prophage 

Bacteriophages o Filamentous Phages  Single strand DNA  Used to produce single stranded recombinant DNA  Looks like long fibers  Causes productive infections  Host cells not killed, just grow slowly  M13 phages as model

4

 Attaches to protein on F pilus of E Coli  Single strand DNA genome enters cytoplasm o DNA polymerase synthesizes complementary strand  Called replicative form (RF): one strand used as template for synthesis of mRNA, copies of genome  M13 phage coat protein inserted into cytoplasmic membrane  Other proteins cause pores  As phage DNA is excreted through pores, coat proteins coat DNA, for nucleocapsids 

Roles of Bacteriophages in Horizontal Gene Transfer o Generalized transduction  Results from packaging error during phage assembly  Some phages degrade host chromosome; fragments can be mistakenly packaged into phage head  Cannot direct phage replication cycle  Termed generalized transducing particles  Following release, can bind to new host, inject DNA  DNA may integrate via homologous recombination, replacing host DNA  Any gene from donor cell can be transferred o Specialized Transduction  Excision mistake during transition from lysogenic to lytic cycle of temperate phage  Short piece of flanking bacterial DNA removed; piece of phage DNA remains

5

 Excised NDA incorporated into phage heads; defective particles released  Can bind to new host, inject DNA  Bacterial genes may integrate via homologous recombination  Only bacterial genes adjacent to integrated phage DNA transferred 

Bacterial defenses against phages o Several approaches bacteria can take o Preventing phage attachment  Alter or cover specific receptors on cell surface  May have other benefits to bacteria  Surface polymers (biofilms) also mask receptor o Restriction-modification systems: two enzymes  Restriction enzymes recognize, cut short nucleotide sequences  Bacteria have different versions, hundreds of varieties  Modification enzymes methylate host sequences normally recognized by restriction enzymes; protect  Restriction enzymes now do not recognize  Enzymes occasionally methylate phages DNA, allow infection o CRISPR System: recently discovered clusters of regularly interspersed short palindromic repeats  Phage spacer DNA inserted into CRISPR  Transcribed

6

 Small RNAs bind to Cas  Binding of spacer RNA to phage targets phages for destruction 

Methods Used to Study Bacteriophages o Viruses multiply only inside living cells  Must cultivate suitable host cells to grow viruses  Bacterial cells easier than animal cells  Plaque assays used to quantitate phage particles in samples: sewage, seawater, soil  Soft agar with bacterial host and specimen, poured in petri dish  Bacterial lawn forms  Zones of clearing form bacterial lysis are plaques  Counting plaque forming units (PFU) yields titer



Animal Virus Replication o 5 step infection cycle  Attachment: viruses bind to receptors (glycoproteins on plasma membrane), more than one bind required, normal function unrelated to viral infection, specific receptors required (limits range of virus)  Penetration and uncoating: fusion or endocytosis, naked viruses cannot fuse  Synthesis: expression of viral genes to produce viral structural and catalytic genes, synthesis of multiple copies of genome, most DNA viruses multiply in nucleus, enter through nuclear pores  3 general replication strategies depending on type of genome of virus

7

o Replication of DNA viruses: usually in nucleus, poxviruses are exception (replicate in cytoplasm), dsDNA rep. straightforward, ssDNA similar except complement first synthesized o Replication of RNA viruses: single stranded, replicate in cytoplasm, require RNA polymerase allows antigenic drift o Replication of reverse-transcribing viruses: encode reverse transcriptase, makes DNA from RNA  Assembly: protein capsid forms, genome enzymes packaged, takes place in nucleus or cytoplasm  Release: viral protein spikes insert into host cell membrane, covered with matrix protein and lipid envelope, naked viruses released when host cell dies 

Categories of animal Viruses Infection o Acute and Persistent Infections  Acute: rapid and onset, short duration  Persistent: continue for years, may or may not have symptoms  Some viruses exhibit both (ex: HIV)  Persistent infection chronic or latent  Chronic: continuous production of low levels of virus particles  Latent: viral genome (provirus) remains silent in host cell, can reactivate, provirus integrated into host chromosome or replicate separately, much like plasmid, cannot be eliminated



8

Ss(+) rna can be used for protein synthesis, while ss(-) rna cannot



Naked viruses don’t go into the cell, only their DNA does



Enveloped viruses attaches to the cell and gets engulfed into the cell



Viruses and Human Tumors o Tumor is an abnormal growth  Cancerous or malignant can metastasize, benign do not  Proto-oncogenes and tumor suppressor genes work together to stimulate, inhibit growth and cell division  Mutations cause abnormal and/or uncontrolled growth  Viral oncogenes similar to host proto-oncogenes, can interfere with host control mechanisms, induce tumors



o Productive infections, latent infections, tumors  Virus-induced tumors are rare, most result from mutations 

Cultivating and Quantitating Animal Viruses o Viruses must be grown in appropriate host  Done by inoculating live animals  Cell culture or tissue culture commonly used o Effects or Viral Replication of Cell Cultures  Many viruses cause distinct morphological alterations called cytopathic effect

9

 Cells may change shape, fuse, detach from surface, lyse, fuse, into giant multinuclear cell (syncytium) or form inclusion body (site of viral replication) o Quantitating Animal Viruses  Plaque assays using monolayer of tissue culture cells  Direct counts via EM  Quantal assay: dilution yielding  Hem agglutination 

Plant Viruses o Do not attach to cell receptors, enter via wounds in cell wall, spread through cell openings o Plants rarely recover, lack specific immunity o Many viruses’ extremely hardy o Transmitted by soil, humans, insects, seeds, tubers, pollen, etc.



Other Infectious Agents: Viroids and Prions o Viroids  Small, single stranded RNA  Forms closed ring, hydrogen bonds  Found only in plants, enter through wound sites o Prions  Proteinaceous infectious agents  Composed solely of protein, no nucleic acids  Linked to slow, fatal human and animal diseases  Usually transmissible only within species  Ex: mad cow disease  Prion proteins accumulate in neural tissues

10

 Neurons die  Tissue develop holes  Brain function deteriorates  Cell produce normal form  PrPc (prion protein)  Proteases readily destroy  Infectious prion proteins  PrPsc (prion protein)  Resistant to proteases  Resistant to heat and chemical treatment  Hypothesizes that prpsc converts to prpc folding to prpsc



11...