Bio 240 Chapter 6 notes!
Bio 240 Chapter 6 notes! Bio 240
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This 8 page Class Notes was uploaded by Izabella Nill Gomez on Saturday October 10, 2015. The Class Notes belongs to Bio 240 at University of Tennessee - Knoxville taught by Dr. Hughes in Summer 2015. Since its upload, it has received 27 views. For similar materials see General Genetics (Bio 240) in Biology at University of Tennessee - Knoxville.
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Date Created: 10/10/15
Biology 240 Chapter 6 notes Bacteria and their viruses have extremely short reproductive cycles They can also be studied in pure culturesinge species or mutant strain of bacteria or one type of virus can be isolated and investigated independently of other similar organisms Bacteria are essential to studies due to their easy manipulation and generation of solid results for researchers Genetically homogeneous bacteria cultures can give rise to cells with inheritable variation under unique environmental conditions Adaptation hypothesis implies that the interaction fo the phage and bacterium is essential to acquisition of immunity Exposure quotinducesquot resistance Spontaneous mutations occur regardless of the presence or absence of bacteria phage T1 is another alternative to the resistance of E coli Fluctuation tests mark initiations of modern bacterial genetic studies Mutant cells that arise can be isolated by selection culturing organisms under conditions where only the desired mutants grow well Bacteria are usually haploid so all mutations are directly expressed in the descendants Bacteria are grown either in liquid or a semisolid agar surface Minimal mediums contain only one organic carbon source Z zK glucoselactose various inorganic ions such as ZNa etc can be added as Clquot well To grow on the medium bacteria must be able to synthesize all nutrients being prototrophic wild type The mutant that cannot synthesize one or more nutrients is an auxotroph A complete medium has been extensively supplemented with nutrients an auxotroph might not be able to synthesize If indicated by a minus sign after a nutrientamino acid the auxotroph cannot metabolizesynthesize the chemical If an antibiotic is present with an r the bacteria is resistant if with an s the bacteria is susceptible and cannot resist the antibiotic and survive The growth pattern of bacteria Lag phase slow Log phase period of rapid exponential growth cells divide continually 9 Stationary phase reached when the cell density is maximized to about 10 cellsmL in a petri dish Genetic recombination here refers to the replacement of one or more genes present in a chromosome of one cell with those from the chromosome of a genetically distinct cell Vertical gene transfer transfer of genetic information within a species Horizontal gene transfer transfer of genetic information between species Plays a role in the evolution of bacteria Those transferred horizontally usually convey a selective advantage Ex antibiotic resistance Conjugation process by which genetic information from one bacterium is transferred to and recombined with that of another bacterium Ex two auxotrophs are mated to create a colony of prototrophs 4 Fquot cells cell donors that give part of their chromosomes F for fertility Fquot cells receive donor chromosome material DNA and recombine with their own chromosome Cel to cell contact is essential for chromosome transfer and genetic recombination This is the initial stage for conjugation and is mediated by the F pilus sex pilus a 69 mm tubular extension of the cell After contact is initiated chromosome transfer is possible The pilus carries DNa from one bacteria to another and is easily broken affecting the transfer of DNA partial transfer occurs 4 Fquot cells have a Fertility factor that confers the ability to donate a part of their chromosome during conjugation Certain environmental factors eliminate the F factor from fertile cells However if quotinfertilequot cells are grown with fertile donor 4 4 cells the F factor is regained The F factor is mobile Fquot becomes Fquot and the F factor is passed to all recipient cells The F factor consists of a circular double stranded DNA molecule equivalent to 2 of the chromosome and has 40 genes tra genes transfer genetic information involving genes for formation of the pilus since 2 Fquot cannot form a pilus on its own The transfer of the F factor involves the separation of one strand to move into the recipient Both are replicated F factor is really an autonomous genetic unit called the plasmid DNA that is transferred to the receiving bacterial cells are nonfunctional unless they are incorporated into the bacterial chromosome Proteins mediate this recombination Hfr high frequency recombination cells undergo recombination 1000x more than 4 the F factor cells alone and constitute a special class of Fquot cells If the donor 4 cell is Hfr recipients never become Hfr and remain Fquot Hfr bacteria are produced by the spontaneous integration of a plasmid with transposable elements DNA oaters The plasmid recognizes the site can integrate into the bacterial chromosome and still exchange information to Hfr and bacterial genes Conjugation z is usually interrupted between Fquot and Hfr before the chromosome transfer is complete Z I I FL reCIpent turns F low recombination rate 4 HfrgtIltFquot recipient remains Fquot high recombination rate Hfr contains a nonrandom pattern of gene transfer but varies Hfr is helpful for chromosome mapping of E coli Interrupted mating technique sets two strains in a blender and interrupts mating at certain intervals Demonstrated that depending on the Hfr strain certain genes are transferred and recombine sooner than others Chromosome of Hfr bacterium is transferred linearly so gene order and distance between genes can be predicted through time mapping which uses time to measure recombination Order in which Hfr genes are donated to a recipient vary The E coli chromosome is circular because the point of origin transfer varied In various Hfr strains the F factor integrated into the chromosome at different points and the position determined the origin 0 site Conjugation rarely if ever lasts long enough to allow the chromosome to pass through conjugation tube leaving recipient cells from Hfr to 4 F6 Z ZgtIltFquot F1 receives F factor but no recombination occurs the F factor can lose its integrated status When this occurs the F factor frequently Z carries several adjacent genes with itdenoted F a special case of Fquot F 4 4 behaves like Fquot by initiating conjugation with Fquot when this occurs the F 4 factor is transferred to Fquot and replicated making duplicates of genes because the recipient still has a whole chromosomethis creates a merozygote Rec genes play an essential role in recombination mutants diminish The gene product Rec A protein play an important role in recombination with single stranded DNA or the linear end of a double stranded DNA that has unwound Singlestrand displacement is a common form of recombination in bacterial species Rec A facilitates recombination of a homologous region of the host chromosome When double stranded DNA enters the recipient cell one stand is often degraded leaving the other complementary as a source of recombination Must nd homologous region along the host and the Rec A facilitates recombination Rec BCD protein is an enzyme consisting of polypeptide subunits encoded by 3 other rec genes Important for when double stranded DNA is a source of genetic recombination Rec BCD unwinds the helix facilitating recombination with Rec A Rec A and BCDunwind DNA to make it single stranded degrade and knick the pieces Rec A is loaded onto the molecule BCD disassembles and allows rec A to create a crossing over double stranded heteroduplex for repair and integration of genes Plasmids Ex F factors Often exist in multiple copies in the cytoplasm each may contain one or more genes and often have a few Replication depends on the same enzymes that replicate the chromosome of the host cell and are distributed to daughter cells along with the host chromosome during cell division Many plasmids are con ned to the cytoplasm Others such as the F factor can integrate into the host plasmids that can exist autonomously or integrate into host episomes Plasmids can infect different bacteria and pick up pieces of DNA and send to other bacteria this is how antibiotic genes are spread Some genes are born by plasmids ex F factor toxin gene R plasmids consist of resistant transfer factors RTF and one or more determinants RTF encoded genetic information essential to transferring the plasmid between bacteria R determinants are genes conferring resistance to antibiotics or heavy metals such as mercury There are many similar RTFs but not rdeterminants each speci c to one antibiotic There can be resistance to many or several antibiotics However there can be many rdeterminants but not RTFs information cannot be transferred Col Plasmid ColEl from E Coli Encodes 1 or more porteints highly toxic to bacterial strains that do not have the same plasmid Colicin protein can kill neighboring bacteria and bacteria carrying the plasmid are coicinogenic Col plasmids also have gene encoding immunity proteins that protect host cells from the toxin Col plasmids are usually not transmissible Transformation another mechanism for recombining genetic information in some bacteria Small pieces of extracellular DNA is taken up by living bacterium potentially leaving to stable genetic change in the recipient cell Also allows for incorporation of antibiotic resistance Important to bacterial survival special receptors allow for DNA to enter cell Double stranded DNA crosses the complex and one strand is degraded by nucleases DNA entering the cell is single stranded integrated with chromosomal DNA through rec A Can be used to map genes although more limitedly than conjugation PFOCESSE 1 Entry of foreign DNA into recipient cell 2 Recombination between foreign DNA and homologous region in recipient chromosome The rst step can occur without the second step although it will not lead to recombination results in addition of foreign DNA to bacterial cytoplasm but not chromosome Competence physiological state necessary to take up DNA Various kinds of bacteria undergo transformation naturally and others can be induced Entry of DNA occurs at limited number of receptor sites on the surface of competent cells Passage through cell is an active process that requires energyspeci c transport molecules Substances that inhibit energy production or protein synthesis inhibit transformationfor recombination to be detected the transforming DNA must be derived from a different strain of bacteria that bears variation mutation Once integrated the recombinant region contains one host present originally mutant start Heteroduplex is the region named after this Mismatch of base sequences activates the repair process following repair and DNA replication once chromosome has the original recipient sequence and one with the mutant Following division one nonmutant cell and mutant cell is produced Cotransformation of several genes can occur simultaneously Genes close enough to each other are linked linkage here is the proximity of genes to permit co transformation If not linked the simultaneous transformation occurs only as a result of two independent events involving two distinct segments of DNA chances are low Bacteriophagesphages viruses that have bacteria as a host Bacterial reproduction of phages can lead to another mode of recombination transduction Bacteriophage T4 is a group of relates viruses referred as Teven phages lts DNA is contained by an icosahedral 20 faced protein coat and can encode more than 150 average genes The tail bers contain binding sites in tips that recognize unique areas of the outer surface of the cell wall of most E coli The life cycle of page T4 begins when the virus binds to the bacterial host cell by adsorption Then ATPdriven contraction of the tail causes the core to penetrate the cell wall and the DNA enters the host DNA RNA and protein synthesis is inhibited from the host and sunthesis of viral molecules begins At the same time degradation of the host DNA occurs Infection period is intense viral gene activity Initially the phage DNA replication occurs leading to a pool of viral DNA then body is synthesized the host cell lysed and phages released 1 DNA packaging as viral heads assembled 2 Tail assembly 3 Tail ber assembly Approximately 200 new viruses are constructed Phage gene product lysozyme ruptures the cell through lysis and completes the lytic cycle Plaque assay determines the number of bacteriophages that are produced during infection Plaque occurs wherever a single virus initially infected one bacterium in the culture that has grown during recombination Represents clones of single infecting bacteriophages If the dilution factor is low plaques are plentiful lysing a whole lawn of bacteria lf dilution is increased plaques can be counted The initial phage densityplaque mL x dilution factor Essential in mutational and recombinational studies of bacteriophages lnfection of a phage does not always result in viral reproductions and lysisviruses can coexist with bacteria Upon entry viral DNA is integrated into the host chromosome instead of replicating in the bacterial cytoplasmysogeny So for each of the bacterial chromosomes replicated viral DNA is also replicated and passed on No new viruses are produced no lysis Under certain circumstance however it can occur ex ultraviolet radiation or light treatment Prophage viral DNA is integrated into the bacterial chromosome Temperate phages viruses that lyse the cell or behave as prophages Virulent phages only lyse bacteria The number of bacterium that harbors a prophage is lysogenized and lysogenic capable of being lysed as a result of induced viral reproduction Viral DNA is an episome that can replicate either in the cytoplasm of the cell or is part of its chromosome Transduction the process of bacterial recombination mediated by bacteriophages Sometimes a small piece of bacterial DNA can be packaged along with viral chromosomes or instead of it Only a few bacterial genes are present in the transducing phage the ability to transduceinfect is unrelated to the type of DNA in the phage head making the transduction possible When bacterial DNA is injected into the host it either remains in the cytoplasm or recombines with homologous region of the bacterial chromosome If the bacterial DNA remains in the cytoplasm it is transmitted to the progeny but not replicated Only a single cell is partially diploid and is producedabortive transduction lf bacterial DNA recombines with the host complete transduction occurs where transduced genes become a permanent part of the chromosome which is passed on to all daughters Both are subclasses of generalized transduction characterized by the random nature of DNA fragments and genes that are transduced most are the abortive type 1020x less frequent than complete transduction Specialized transduction occurs when transfer of bacterial DNA is not random but only the strainspeci c genes are transduced Occurs when DNA representing temperate phages break out of the host chromosome bringing with it bacterial DNA on either of its ends Transduction is also used for linkage and mapping studies of bacterial chromosomes Cotransduction can occur when the fragment of bacterial DNA involved is large enough to include several genes Transduction studies by studying 2 or 3 linked genes can determine the precise order of those genes Phage mutations often affect the morphology of plaques formed following lysis of bacterial cells In wildtype phages reproduction is inhibited once a particular sized plaque is formed Rlysis r mutants of T2 phages overcome this producing larger plaques The host range h mutants extends the range of bacterial hosts a phage can haveconfers with the ability to adsorb E coli B2 unlike its wild type counterpart Genetic recombination of bacteriophages was discovered during mixed infection experiments in which 2 nutrient strains simultaneously infect the same bacterial culture Mixed infection experiments are designed so that the number of viral particles sufficiently exceeds the number of bacterial cells to ensure simultaneous infection of most cells nu both viral strains If 2 loci are involved recombination is intergenic Ex study of E coli with parental viruses hr wild type locus range rapid lysis or hr extended host range normal lysis genotype If no recombination occurs the parental would be the offspring genotype Recombinants hr and hr are detected in addition to parental genotypes Recombination frequencyhrhrtotal plaques x 100 Similar studies to eukaryotes have been performed Recombination is not restricted to 2 chromosomes As page development progresses chromosomes are randomly removed from the pool and packed into the phage head forming mature particles Thus a variety of parental and recombinant genotypes are represented in the progeny Possible exam questions 1 What are the possible mediums prototrophs can inhabit minimal and complete 2 Recombination in bacteria is better with 2 crossovers
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