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 unique properties 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 may take 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 themselves Viruses 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 infection 2 host DNA polymerase is co opted to produce more viral DNA 3 viral proteins are then transcribed translated 4 proteins and DNA are assembled into new virus particles 5 virus particles leave cell often killing it Bacteriophages phages for short these viruses target bacteria they have two different kinds of life cycles 1 lytic cycle characteristic of virulent viruses which always kill their host fig 18 4 life cycle of the T4 phage which infects E coli Ultimate effect is to produce 100 200 phage particles which are released when the bacterial cell lyses and dies 2 lysogenic cycle present in temperate viruses which have the capacity fot two types of cycles lytic as above and lysogenic fig 18 5 life cycle of the l phage also infects E coli lysogenic cycle l phage DNA becomes incorporated in host DNA forming a prophage 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 1 Cells then lyse Animal Viruses There are a number of different types that can be differentiated on the basis of the genetic material and how it is used table 18 1 double stranded dsDNA singlestranded ssDNA double stranded dsRNA single strand ssRNA three types can serve as mRNA can serve as template for mRNA or can serve as template for DNA synthesis Retro viruses have single stranded RNA ssRNA that serves as a template for DNA synthesis this is accomplished by an enzyme called reverse transcriptase Many retroviruses are pathogens including HIV human immunodeficiency virus fig 18 7 shows HIV life cycle Viroids small RNA molecules that are plant pathogens Prions could possibly be infectious proteins mad cow disease in England may be caused 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 Genetics Recall that bacteria do not have nuclei instead the chromosomes are found in a region called the nulceoid Bacteria also have unique structures called plasmids which are small 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 daughter cells 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 in genetic make up that is there may be alternative forms of certain genes called alleles allele alternative gene forms which may code for slightly different products In a population showing genetic variation not all individuals are genetically alike due to the presence of allelic variation Variation is important because it allows for the selection of individuals resulting in changes in the characteristics of a population 3 Sources of genetic variation a mutation many mutations produce defective products other are silent in that they do not produce any appreciable change so called neutral mutations 2 b genetic recombination text definition p 341 is combining of genetic material from two individuals into the genome of a single individual processes very useful in modern molecular biology techniques transformation direct uptake and incorporation of foreign DNA into a bacterial cell transduction phages may carry some bacterial genes after infection these genes may become incorporated into the genome of another bacterium conjugation exchange of genetic material in the form of plasmids between two bacteria Plasmids circular pieces of DNA containing a small number of genes that are not a part of the bacterial chromosome they are self replicating The genes present in plasmids are generally not required for survival of the bacterium Often they contain genes that code for characters which allow the bacterium to survive under certain special conditions 3
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