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FSU BSC 2010 - Lecture Notes

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Topic 15: VIRUSES & BACTERIA: some are pathogens but others have been vital tools in molecular biology research (lecture 24).OBJECTIVES:1. Be able to compare and contrast lytic vs. lysogenic life cycles of bacteriophages.2. Know how animal viruses are categorized and understand what are the uniqueproperties of retroviruses.3. Know the life cycle of HIV.4. Understand the difference between bacterial nucleoids and plasmids.5. Know the importance of bacterial genetic recombination and the three ways it maytake place in prokaryotic cells.Virus- RNA or DNA accompanied by protein which have the capacity to harness the machinery of cells for replication; they are not living per se since they cannot replicate themselvesViruses- common features (fig. 18.2)- genome (consisting of DNA or RNA); capsid(protein coat); viral envelope (membrane cover with glycoproteins).Generalized life cycle of a virus (fig. 18.3)-1. infection2. host DNA polymerase is co-opted to produce more viral DNA3. viral proteins are then transcribed & translated4. proteins and DNA are assembled into new virus particles5. virus particles leave cell; often killing it Bacteriophages (phages for short)- these viruses target bacteria; they have twodifferent kinds of life cycles:1. lytic cycle- characteristic of virulent viruses which always kill their host; fig. 18.4; lifecycle of the T4 phage which infects E. coli. Ultimate effect is to produce 100-200 phageparticles which are released when the bacterial cell lyses and dies.2. lysogenic cycle- present in temperate viruses which have the capacity fot two typesof cycles- lytic as above and lysogenic.fig. 18.5- life cycle of the  phage; also infects E. coli. - lysogenic cycle:  phage DNA becomes incorporated in host DNA forming aprophage; bacterial reproduction results in the production of many daughter cells containing the prophage-lytic cycle: at some point the prophage DNA leaves the bacterial DNA; phage genes become expressed and new virus particles are synthesized.1Cells then lyse.Animal VirusesThere are a number of different types that can be differentiated on the basis of thegenetic material and how it is used (table 18.1)- double-stranded dsDNA, single-stranded ssDNA, double-stranded dsRNA, single-strand ssRNA ( three types- can serveas mRNA, can serve as template for mRNA or can serve as template for DNAsynthesis).Retro viruses- have single-stranded RNA (ssRNA) that serves as a template for DNAsynthesis; this is accomplished by an enzyme called reverse transcriptase. Manyretroviruses are pathogens including HIV (human immunodeficiency virus); fig. 18.7shows HIV life cycle.Viroids- small RNA molecules that are plant pathogensPrions- could possibly be infectious proteins; mad cow disease in England may becaused by a protein which has a defective conformation; it may cause proteins with the normal conformation to assume the abnormal conformation. The resulting proteins are extremely stable, accumulate in brain cells and cause serious pathologies.Bacterial GeneticsRecall that bacteria do not have nuclei; instead the chromosomes are found in a regioncalled the nulceoid. Bacteria also have unique structures called plasmids which aresmall circular pieces of DNA consisting of a much smaller number of genes.Key issues relating to bacterial genetics:1. Generation time- time for cell to replicate its DNA and divide into identical daughtercells; this can be very fast on the order of minutes to several hours. 2. Genetic variation- within a population there may be some individual differences ingenetic make-up; that is, there may be alternative forms of certain genes calledalleles (allele = alternative gene forms which may code for slightly differentproducts). In a population showing genetic variation, not all individuals aregenetically alike due to the presence of allelic variation. Variation is importantbecause it allows for the selection of individuals resulting in changes in thecharacteristics of a population.3. Sources of genetic variation:(a) mutation- many mutations produce defective products; other are silent in thatthey do not produce any appreciable change (so-called neutral mutations)2(b) genetic recombination- text definition (p. 341) is “combining of genetic materialfrom two individuals into the genome of a single individual”; processes veryuseful in modern molecular biology techniques transformation- direct uptake and incorporation of foreign DNA into a bacterialcell.transduction- phages may carry some bacterial genes; after infection thesegenes may become incorporated into the genome of another bacteriumconjugation- exchange of genetic material in the form of plasmids between twobacteriaPlasmids- circular pieces of DNA containing a small number of genes that are not apart of the bacterial chromosome; they are self-replicating. The genes present inplasmids are generally not required for survival of the bacterium. Often they containgenes that code for characters which allow the bacterium to survive under certainspecial conditions.


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FSU BSC 2010 - Lecture Notes

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