PCB3063 Exam 3 Study Guide Chapter 8 Section 8 1 Bacteria Mutate Simultaneously and are Easily Cultured This chapter is about the analysis and mapping of genes in bacteria prokaryotes and bacteriophages viruses that use bacteria as their hosts Bacteria and their viruses are especially useful research organisms in genetics for several reasons They have extremely short reproductive cycles they can reproduce in a short period of time and they can be studied in pure cultures A prototroph can synthesize all essential organic compounds and therefore can be grown on minimal medium Prototrophs are said to be wild type for all growth requirements o Minimal media contains only an organic carbon source for energy glucose or lactose and a variety of ions Na K Mg2 Ca2 and NH4 Minimal media are those that also contain the minimum nutrients possible for colony growth On the other hand if a bacterium loses the ability to synthesize one or more organic components through mutation it is an auxotroph Serial dilutions are used if the number of colonies is too great to count and they 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 millimeter of the initial inoculums used to start the dilution series Section 8 2 Genetic Recombination Occurs in Bacteria It is important to note that genetic information can be transferred from one bacterium to another resulting in an altered genotype Three processes result in this transfer of genetic information conjugation transformation and transduction Knowledge of these processes has helped us understand the origin of genetic variation between members of the same bacterial species and different bacterial species When transfer of genetic information occurs between members of the same species the term vertical gene transfer applies When transfer occurs between members of related but distinct bacterial species the terms horizontal gene transfer applies In 1946 Joshua Lederberg and Edward Tatum showed that bacteria undergo conjugation a process by which genetic information from one bacterium is transferred to and recombined with that of another bacterium Their initial experiments were performed with two multiple auxotrophs nutritional mutants of E Coli strain K12 Each strain was grown separately in supplemented media and then the cells from both were mixed and grown together for several more generations They were then plated on minimal media and the cells that grew on the media were autotrophs It soon became evident that different strains of bacteria are involved in a unidirectional transfer of genetic material When cells serve donors of parts of their chromosomes they are designated F cells F for fertility Recipient bacteria receive the donor chromosome material and recombine it with part of their own chromosome They are designated F cells as as as Conjugation is one of the means of genetic recombination in bacteria Cell contact is essential for chromosome transfer to occur Another experiment done by Bernard Davis showed that if there no direct contact auxotrophs F and F cells do not exchange genetic material and no prototrophs are formed Physical contact is the first step in conjugation established by the F pilus or sex pilus pili The F cells contain a fertility factor F factor that confers the ability to donate part of their chromosome during conjugation F factor is mobile and consists of a circular double stranded DNA molecule containing 19 genes is of An E Coli may or may not contain the F factor When this factor is present the cell is able to form a sex pilus and potentially serve as a donor of genetic information During conjugation a copy the F factor is almost always transferred from the F cell to the F recipient converting the F state 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 An Hfr high frequency recombination strain behaves as donor and is a special class of F cells o F and F recipient becomes F low rate of recombination o Hfr and F recipient remains F high rate of recombination Interrupted matings demonstrated that specific genes in an Hfr strain are transferred and recombined sooner than others An ordered linear transfer of genes is correlated with the length of time conjugation o The gene order and distance between genes could be predicted o This is a basis for first genetic map in bacteria Gene transfer by Hfr strains led to the understanding that the E Coli chromosome is The position of the F factor determines the initial point of transfer between the Hfr and an A merozygote is a partially diploid cell that results from the replication and transfer of proceeded circular F cell the F factor Section 8 3 Rec Proteins are Essential to Bacterial Recombination Researchers looked for several functional gene products present in normal cells but missing in rec mutant cells and showed that they played a role in genetic recombination This first product is called the RecA protein This protein is important plays an important role in recombination involving either a single stranded DNA molecule or the linear end of a double stranded DNA molecule that is unwound As it turns out single stranded displacement is a common form of recombination in many bacterial species The second related gene product is a more complex protein called the RecBCD protein an enzyme consisting of polypeptide subunits encoded by three other rec genes Section 8 4 The F Factor is an Example of a Plasmid Plasmids double stranded closed circle of DNA o exist in multiple copies in the cytoplasm o may contain one or more genes o use the same replication enzymes as host o are distributed to daughter cells o replicate independently of the bacterial chromosome F factor plasmids confer fertility and contain genes for sex pilus formation on which R plasmids consist of two components the resistance transfer factor RTF and one or o RTF encodes genetic information essential to transferring the plasmid between genetic recombination depends more r determinants bacteria o R determinants confer resistance to antibiotics Col plasmids encode colicins that can kill neighboring bacteria Section 8 5 Transformation is Another Process Leading to Genetic Recombination in Bacteria Transformation provides another mechanism for recombining genetic information in some bacteria
View Full Document
Unlocking...