Genetics Test 3 Chapter 8 Introduction Bacteria and viruses have sort reproductive cycles Hundreds of generations give rise to billions of genetically identical bacteria or phages in a very short time Bacteria and viruses can be studied in pure cultures wherein a single species can be isolated and investigated independently of other similar organisms Bacteriophages are viruses that use bacteria as their hosts Their study has resulted in an accumulation of knowledge in many areas of genetics especially molecular genetics recombinational phenomena and gene structure Extensive knowledge of bacteria and their resident plasmid has led to their widespread use in DNA cloning and other recombinant DNA studies 8 1 Bacteria Mutate Spontaneously and Are Easily Cultured The adaptation hypothesis proposes that the interaction of bacteriophage and bacterium is essential to the bacterium s acquisition of immunity to the phage Exposure to the page induces resistance in the bacteria Spontaneous mutation that occurs in the presence or absence of phage is considered the primary source of genetic variation in bacteria Pure cultures of bacteria can give rise to mutant cells that can be isolated and established independently from the parent strain by using selection techniques Containing only a single chromosome bacteria and viruses are haploid Thus all mutations are expressed directly minimal medium Bacteria can be grown in a liquid culture medium or in a petri dish on a semisolid agar surface A prototroph can synthesize all essential organic compounds and therefore can be grown on o Contains only an organic carbon source glucose or lactose and a variety of ions Na K Mg2 Ca2 and NH4 Through mutation and auxotroph has lost the ability to synthesize one or more essential compounds it must be provided with them in the medium if it is too grow Serial dilutions are used to study bacteria quantitatively Since each colony arose from a single bacterium the number of colonies multiplied by the dilution factor represents the number of bacteria in each milliliter of the initial inoculum used to start the dilution series 8 2 Genetic Recombination Occurs in Bacteria Genetic info can be transferred from one bacterium to another resulting in an altered genotype Three processes result in this transfer of genetic info o Conjugation transformation and transduction Vertical gene transfer applies to the transfer of genetic info between members of the same species Horizontal gene transfer refers to the transfer of genetic info between members of related but o Usually confers survival advantages that is transfer of antibiotic resistance genes to distinct bacterial species another species o Major factor in speciation Conjugation in bacteria the discovery of the F and F strains o Bacteria undergo conjugation in which Genetic info from one bacterium is transferred to another It recombines with the second bacterium s DNA Different strains of bacteria are involved in a unidirectional transfer of genetic material genes cells Cells serving as donors of parts of their chromosomes are designated F cells F for fertility Recipient bacteria receive the donor DNA and recombine it with part of their own chromosome and are designated as F cells Cell contact is essential for chromosome transfer to occur o Physical contact is the first stem in conjugation establishes by the F pilus sex pilus F cells contain a fertility factor F factor that confers the ability to donate part of their chromosome during conjugation o F factor is mobile and consists of a circular double stranded DNA molecule containing 19 o One strand of the double helix moves into the recipient cell via the sex pilus and the other one remains Both re form their double helix and become F E coli may or may not contain the F factor When the F factor is present the cell is able to form a sex pilus and serve as a donor of genetic info During conjugation a copy of the F factor is transferred from the F cell to the F recipient converting the recipient to the F state Hfr bacteria and chromosome mapping o An Hfr high frequency recombination strain behaves as donor and is a special class of F o And Hfr strain can donate genetic info to an F cell but the recipient does not become F F x F recipient becomes F low rate of recombination Hfr x F recipient remains F high rate of recombination o Interrupted matings demonstrated that specific genes in an Hfr strain are transferred and recombined sooner than others o An ordered linear transfer of genes is correlated with the length of time conjugation proceeded The gene order and distance between genes could be predicted Basis for first genetic map in bacteria Gene transfer by Hfr strains led to the understanding the E coli chromosome is circular During conjugation between an Hfr and an F cell the position of the F factor determines the initial point of transfer o Conjugation rarely allows the entire chromosome to ass across the conjugation tube This procedure has established the location of approximately 1000 genes Recombination in F x F matings a reexamination o When F and F cells are mixed conjugation occurs readily and each F cell involved in conjugation with an F cell receives a copy of the F factor no genetic recombination occurs o At an extremely low frequency in a population of F cells the F factor integrates spontaneously from the cytoplasm to a random point in the bacterial chromosome converting the F cells to the Hfr state o In F x F matings the low frequency of genetic recombination is attributed to the rare newly formed Hfr cells which undergo conjugation with F cells o Since integration of the F factor is random gene transfer is also random within the larger F F population The recipient bacteria is a recombinant but will remain F but upon conjugation The F state and merozygotes with an F cell will before F o In some cases and F factor is excised from the chromosome of an Hfr strain and reverts to the F state o In the process the F factor referred to as F often brings several adjoining genes with it o Transfer of an F to an F cell results in a partially diploid cell called merozygote 8 3 Rec Proteins Are Essential to Bacterial Recombination Genetic recombination is a regulated process in bacteria like eukaryotes Requires functional gene products from RecA and RecBCD Bacterial mutants deficient in any of these components don t undergo recombination 8 4 The F Factor Is an Example of a Plasmid Plasmids are composed of a double
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