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Download Genetics of Viruses & Bacteria
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Genetics of Viruses & Bacteria Viruses Nucleic acid in protein coat Infectious Discovered through studies of tobacco mosaic disease Viruses DNA virus or RNA virus Can be single or double stranded Linear or circular nucleic acid Viruses – protein shell that encloses genome Made of protein subunits (capsomeres) Accessory structures to enhance infecting ability Viral envelope – membrane from host cell covering capsid Capsid Viruses Bacteriophage (phage) Virus that infects bacteria Capsid has polyhedral head & tail attached Reproduce only within host cell Host range: types of cells viruses infect Based on recognition of receptors Tissue specific Viral Infection Depends on virus type DNA viruses – DNA polymerase of host cell to make viral template & new genome RNA viruses – viral-encoded polymerases use RNA as template Both divert resources from host Reproduce in two ways: 1) Lytic Cycle DNA viruses Phage reproductive cycle host death If only this way phage is “virulent phage” Virus docks with receptors injects DNA kills host DNA makes copies bursts out of cell 2) Lysogenic Cycle Replicates genome without host death Can do both “temperate phage” DNA incorporates into host DNA It is reproduced as host reproduces When chromosomes circle up, triggers lytic cycle LYTIC AND LYSOGENIC CYCLES Which is this? Animal Virus Replication – viral DNA inserts into host genome for replication (may activate or inactivate throughout organism’s life) RNA viruses Retrovirus Enzyme – reverse transcriptase RNA used as template to make Provirus Animal Virus Replication DNA integrates (provirus) Host transcribes viral DNA into mRNA & RNA for new infection sites Example: HIV FYI…Other Infectious Agents Prion Infectious protein Misfolded Causes cell mistakes in regular proteins in Bacteria One circular DNA molecule (double stranded) Chromosome packed in one region – nucleoid Also have plasmids Binary fission for reproduction Mutation & genetic recombinations increase diversity Genetic Recombination 3 steps 1) Transformation Alteration of bacterial cell’s genotype by uptake naked, foreign DNA from surrounding environment by surface proteins Genetic Recombination Foreign allele incorporated into bacterial chromosome Now have recombinant Genetic Recombination 2) Transduction Phages carry bacterial genes from one host cell to another Two types Generalized Specialized Genetic Recombination 3) Conjugation Direct transfer of genetic material between two bacterial cells that are joined One way (“Male” to “Female”) “Male” hooks “female” & donates DNA Male has special DNA (F factor) within chromosome or as plasmid Plasmids Small, circular, self-replicating DNA molecule separate from bacterial chromosomes Beneficial for recombination; not necessary for survival R plasmid allows bacteria to be antibiotic resistant Transposons Transposable genetic elements “Jumping genes” Section or copy of section moves within genome Results in recombination Bacterial Gene Expression Adjust enzymatic activity by use of operons Operons Operator Located within promoter (where RNA polymerase binds) or between promoter & coding genes Controls access of RNA polymerase to the genes (so controls if gene is on) Operator + Promoter + Genes (DNA to make enzymes) = Operon Operons Operon switched off by protein called repressor Binds to operator; blocks RNA polymerase Reversible Corepressor: small molecule cooperating with repressor to turn operon off Trp Operon Function – make tryptophan when none is present If tryptophan is present… No need to make trp Repressor is activated by the trp (corepressor) Operon is blocked and switched off Opposite if trp is absent Trp operon Trp Operon Example of repressible operon Transcription inhibited when repressor binds allosterically to regulatory protein Lac Operon Example of inducible operon Stimulated when binding of inducer to repressor Inactivates the repressor Lac operon Lac Operon Quiz Lac operon Function – make enzymes to break down lactose and allolactose If lactose is present… Need to make enzymes to break it down Lactose & allolactose (inducers) binds to repressor to inactivate it Operon works & transcription occurs Opposite if lactose is absent Lac Operon Operons switched on by presence of another molecule (“volume control”) Ex) cAMP