Lecture 18 Comparing Lytic and Lysogenic pathways main difference o Eukaryotes cells when they are infected by the virus they are capable of continuously producing virions while when a bacteria phage enters the lytic cycle it ruptures and dies No bacteria phage that allows the bacteria cell to live during the lytic cycle Lysogenic cycle o 1 Integration step DNA must be integrated into the host chromosome many times by direct recombination Occurs by sit specific non reciprocal recombination o 2 Results in stable home for viral genome Lambda is a prophage and replicates with the chromosome can go through life cycle Lambda no phenotypic change except for it is a repressor doesn t allow infection twice by the same phage o 3 No cell lysis no replication of new phage particles o 4 Excision of prophage may occur lytic cycle initiated Bacteriophage replication o 1 Lytic cycle virulent phage Bacteriophage is replicated 1 200 Host cell is lysed Phage that perform only this type of cycle are called lytic or virulent phage T4 an e coli phage is an example Figure o Green line productive phage infection lytic growth infected the culture with 100 phage over time the number of phage is decreasing why because they are infecting the cells number phage decreasing because they are infecting the cells number of phage available to cells goes down once all phage are absorbed by cells the phage enters the lytic cycle and the number of phage increases dramatically burst in this graph you can see that this is about a hundred times more phage by the end each infected cell at first around 100 replicated about 100 phages o No cell control line without cells the phage can not replicate no increase in number o No adsorption phage cannot replicate o Adsorption but no lysis absorbing into cells that are capable of destroying phage genome number of phage continuously goes down o Period when you first introduce the phage into the sample and before the burst is called the eclipse o Once again you can measure the burst size by looking at the chart o At what point in this curve would you first see cell lysis After 20 minutes Why At 0 minutes the phage are still starting to infect it takes time to replicate their chromosome at 40 minutes a lot of the phage have already lysed at 20 minutes is approx is where the most lysis is happening the phage are now rupturing out of the cell increase in number Bacteriophage replication effect on host o Lytic cycle host cell is lysed Mistake in virion assembly give rise to a virion that can transfer bacterial genes to a second host generalized transduction o Lysogenic cycle genotypic change once the phage integrates into the chromosome you have a genotypic change if genes are expressed then you d have a phenotypic change one change with the lambda is resistance to lambda Mistake in excision transfer of genes to second host specialized transduction Specialized transduction lambda example o Lambda DNA integrates into a special spot on the chromosome everytime same o Cell replicates producing replicates also have the chromosome with the lambda spot phage DNA in it o Then some sort of environmental signal occurs and the lambda phage excises itself from the host chromosome normal circumstance o Now MISTAKE specialized transduction when the lamda phage excises itself it mistakenly takes some of the host chromosome with it The phage replicate with the host chromosome in it Then there are phages called transducing phages that burst out these phages have part phage DNA and part host cell chromosome DNA This special phage can infect a cell and it may have DNA chromosome of the host cell it is going into for example gene Z if this host cell has a defected Z gene this phage now with the Z gene repair the mutated Gene Z on the host chromosome involves the phage picking up genes adjacent to wear the phage integrates into the host In generalized transduction the DNA got chopped up randomly and some phage took up some of this randomly chopped up host DNA chromosome not specifically taken up like specialized transduction Q Host Defense Against Phage o Answer 1 Destroying phage DNA upon entry function of restriction enzymes AND Changing the surface proteins have receptors for the phage Antibiotics are used against bacteria not phage or viruses Bacterial defenses against viral infection o Nucleases endo and exo DNases and Rnases o All bacteria have nucleases that attack DNA DNases and RNA RNases o Exonucleases attack free 5 or 3 ends of DNA RNA molecules o Endonucleases restriction enzymes cleave DNA at specific sequences Host cell DNA protected by methylation of a base in this sequence Viral DNA not methylated not protected BIG business selling purified restriction enzymes for biotechnology industry o Cell surfaces possibility of mutation to alter receptor Viruses must attach to some specific cell surface protein or polysaccharide These are specified by genes which can be mutated Can phage be effective weapons against bacterial disease Frequency of resistant bacterial strains is too high any virus quickly becomes useless as resistant survivors propagated There are some studies where phage is used for treatment Lysogenic conversion by a filamentous phage encoding cholera toxin watch if have extra time at 27 30 Eukaryotic viruses getting into cells into the cell o 1 Eukaryotic phage comes in contact with cell surface and it injects its genome This one is most similar to bacteria phage infecting bacteria cell other two is specific to eukaryotes o 2 Membranes of the envelope of the phage and the cell merge releasing the protein coded phage into the cytoplasm o 3 Endocytosis virus and the eukaryotic cell meet wrapped in membrane endocytosis vesicle around the phage some signal is given to release the phage particle o dsDNA virus ds DNA injected into the cell must travel to the nucleus it can then use the host replication transcription machinery to replicate itself and make mRNA The mRNA can then travel out of the nucleus for translation into a protein o ssDNA virus injected into cell moves into nucleus copied using host replication machinery this can be used to make mRNA that is translated black strand is the one that is used for subsequent infection Genome Replication o ssRNA virus this basically is already mRNA it does not need to go into the nucleus it can begin immediate translation it also can use polymerases to make a complementary copy which is used to make more positive strands to be further