Class Note for MIC 205A at UA
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Date Created: 02/06/15
Microbial Genetics Gen etic tran sfer and recombin ation Genetic recombination I Genetic recombination through sexual reproduction is an important means ofvariation in eukaryotes I Prokaryotes do not have an equivalent process of sexual reproduction I However prokaryotes do have mechanisms by which DNA can be transferred between strains ofthe same species or even between different species I Contributes to a population39s genetic diversity Gene transfer I Vertical gene transfer I Occurs during reproduction Transfer of genes from an organism to its offspring I Horizontal gene transfer I Transfer of genes from one organism to another within the same generation Horizontal gene transfer I Involves one way transfer from a donor cell to a recipient cell l A recipient cell that incorporates DNA from the donor is called a recombinant I Genes are transferred naturally between bacteria by three mechanisms Transtbrmatmn DNA istransferred as naked DNA Cbmugatmn DNA istransferred between baetena thatare in untactvvith eaeb uther Transductiun DNA is transferred by a ba eriai Wu bameriuphage Homologous recombination I DNA introduced into bacteria usually does not have a mechanism to replicate itself I It relies on integration into the genome ofthe host bacterium in order to survive and be passed on incamne DNA Hastgename IImI R ecom mm EEnome I Griffith s experiment 1928 Streptococcus pneumoniae Naked DNA I When a bacterial cell lyses it releases its DNA into the environment Transformation I DNA is transferred as naked DNA DNA breaks into pieces on cell lysis DNA is taken up bythe recipient cell A region of tne recipient DNA is replaced by tne donor DNA recombination I Unrecombined DNA is degraded Competence I Transformation occurs naturally in very few genera of bacteria I Bacillus Haemnphilus Neisseria Acinelnbacler and sorne strains of streptococcus and Staphylococcus I The recipient cell must be in a physiological state in which it can take up DNA it must be competent I Competence alterations in the bacterial cell wall that make it permeable to large DNA molecules I Some bacteria which are not normally competent can be rnade so in laboratory Conjugation i ic39z I DNA transfer between two bacteria that are in contact with one another I Contact between donor and recipient cells is initiated by sex pili I DNA is transfer through a conjugation bridge or open pore between donor and recipient cell I Mediated by a plasmid called an Ffactor fertility factor or a conjugative plasmid Plasmids I Small circular molecules of DNA Replicate independently of the chromosome Usually dispensable for growth but under some conditions provide a selective advantage such as antibiotic resistance or a unique metabolic pathway 7 Conjugative plasmids carry quotWWquot quot39 genes for conjugation including sex pili Mechanism of conjugation I Donor contacts recipient attaches using sex pilus I Ffactor initiates transfer ofa copy of itself I Recipient is converted to a new donor cell J Li s g m Importance of plasmids Hfr cells can transfer chromosomal genes I Providing a selective advantage I Scrne Pseudomonassp have piasrnid enccided enzymestu degrade petruleum aHEIerig tnerntci live Sometimes the Ham in fueltariks cirtdei spiiis will We me We me I Enhancing pathogenicity h 9 h d I The E mii strain causing intant Drtraveler s diarrnea c r m 5 me 9 quot quot carries plasmidsturtuxin pruductiun and bacterial cell creating an Hfr high af 35 m5 frequency recombination I Antibiotic resistance co I Many antipicitic resistance genes are carried an plasmids which can be rapidlytransferred tn either I The recipient cell can bacteria resulting in Widespread resistance tn receive new chromosomal antlblutlcs arid strains that resistant tEI multiple antipiutics such as mEthlElHlncrESlStant genes from a donor Hfr Staphylococcus aureus cir Gulderi Staph cs Transduction Generalized transduction I Mediated by a bacterial virus 39 0010quot CSquot is Weth With bacteriophage or phage SgngggNA is I I DNAfrorri the donor is transferred to the incorporated into the recipient inside the phage particle phage transducing Phage I The donor cell lyses I The transducing phage I Two types oftransduction I Generalized I Specialized infects the recipient cell and injects the donor DNA I DNA integrated into the genome Specialized transduction Transposons I Segments cit DNA tnat can mDVE trcirn cine regicin cit re nemnmimgxmr DNA tEI ancitner and a integrate thruugh HEIHV u ncirnciicigcius reccirnpinaticin I ccintain intcirrnaticin turtneir EIer transpcisiticin Transpusase enzyme icii cutting and ieseaiing DNA Shunleiminaliepealswhich iimmnwi w lhelvanspusase iecdgnizes as Yecumbinatiun snes I inserticin seguences are tne sirnpiesttranspciscins I Cumplextranspusuns carry citner genes e g aritlbl tlc resistance genes Surne phage can integrate intu the ncist ceii gencirne prcipnage Rarely tne prcipnage WlH excise trcirntne gencirne and entera lytic cycle Sumetimes during excisicin tne phagEWill take a pcirticin cit dcincir DNA Witn it and package it inside tne pnage The dcincir ceii lyses The transddcing pnage intects tne recipient ceii and iniects tne dcincir DNA aicing Witn its EIer DNA integrated intci tne gencirne Natural history of a transposon chromosome transposon IO plasmid Genomes of bacteria are elastic I Bacterial genomes are often receiving genetic information from other bacteria through genetic transfer and recombination I In the same way that mutations can be beneficial neutral or harmful so is the recombination of incoming DNA I New gene combinations are maintained if they provide the organism with a selective advantage
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