1 PCB 3063 with Dr Bass Exam 3 Study Guide Ch 8 12 Chapter 8 Prokaryotic Genetics Bacteriophages viruses that use bacteria as their host Bacteria and viruses are extremely useful for research because of their short reproductive cycle and are studied in pure cultures 8 1 Bacteria Mutations and Cultures Bacteria are haploid all mutations are expressed in offspring Mutant cells can be isolated and established independently from parent strain Bacteria are grown in a liquid culture medium or in a petri dish on semisolid agar Minimal medium organic carbon source and a variety of ions Na K Ca Mg 2 NH4 used to show WILD TYPE microorganisms Prototroph is a bacterium that is able to synthesize all essential organic compounds Auxotroph is a bacterium that has lost ability to synthesize one or more organic components through a mutation Ex if a bacteria loses ability to make amino acid histamine then its called his To study bacteria quantitatively we use liquid culture mediums Serial dilutions is a technique used when the number of colonies is too great by using a series of successive dilutions of the original liquid culture until the number of colonies is reduced 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 Recombination is the replacement of one or more genes present in the chromosome of o one cell alters the genotype 3 processes that result in the transfer of genetic info in prokaryotes conjugation transformation and transduction Vertical gene transfer when a transfer of genetic info occurs between same species Horizontal gene transfer when members of distinct but related species exchange genetic info Conjugation with another Process by which genetic info from one bacteria is transferred to and recombined Lederberg and Tatum experiment two multiple autotrophs of E Coli strain K12 Strain A required methionine met and biotin bio in order to grow Strain B required threonine thr leucine leu and thiamine thi Grown separately in supplemented media then mixed together Assumed that any prototroph recovered must have arisen as a result of some genetic exchange and recombination between the mutant strains F cells were chromosome donors whereas the recipient cell was designated F F crossed with a F yields F CELL CONTACT IS NECESSARY 2 Physical interaction is established with the f pilus or sex pilus which is a tubular extension of the cell Fertility factor confers the ability to donate part of their chromosome during conjugation is a mobile element Transfer of f factor involves separation of two strands of DNA and movement of one strand into the recipient Autosomal genetic unit called a plasmid of F cells become F proceeded circular An Hfr high frequency recombination strain behaves as donor and is a special class An Hfr strain can donate genetic information to an F cell but the recipient does not F x F recipient becomes F low rate of recombination Hfr x 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 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 that the E coli chromosome is During conjugation between an Hfr and an F cell the position of the F factor determines Conjugation rarely allows the entire chromosome to pass across the conjugation the initial point of transfer tube This procedure has established the location of approximately 1000 genes 8 3 Rec Proteins How does donor DNA replace a homologous region on the recipient chromosome Major discoveries were made by isolating a group of mutations that impaired the recombination process Researchers isolated several functional gene products in normal cells that were missing in rec mutant cells The first product was RecA protein plays an important role in recombination of single stranded DNA or the linear end double stranded DNA that has unwound Single stranded displacement is a common form of recombination 2nd gene product was RecBCD protein an enzyme consisting of polypeptide subunits encoded by three other rec genes unwinds double helix and facilitates recombination 3 8 4 Plasmids Extra chromosomal hereditary unit in a closed circle form are plasmids Contain one or more genes Replication depends on the same enzymes that replicate the chromosome on the R plasmids consist of two components Classified according to the genetic info specified by their DNA Distributed to daughter cells and exist in multiple copies in the cytoplasm The resistance transfer factor RTF and r determinants RTF encodes genetic information essential to transferring the plasmid between host cell bacteria R determinants confer resistance to antibiotics Col Plasmid is distinct from the R plasmid It encodes one or more proteins that are highly toxic to bacterial strains that do not harbor the same plasmid These proteins are called colicins and can kill neighboring bacteria 8 5 Transformation bacteria Transformation is another mechanism for recombining genetic information in some Small pieces of extracellular DNA are taken up by the living bacteria potentially leading to a stable change in the genes of the recipient cell Transformation process has many steps divided into two categories 1 entry of DNA into recipient cell 2 and recombination of the donor DNA with its homologous region in the recipient chromosome Only cells in the state of competence can take up DNA entry is through a limited number of receptor sites After entry one strand of DNA is digested by nucleases leaving one strand to participate in transformation Once it is integrated into the host strand the helical region where both strands exist from different sources is called the heteroduplex In prokaryotes linkage refers to the proximity of genes that permits cotransformation 8 6 Bacteriophages and Bacterial Viruses Transduction is understood through the genetics of bacteriophages which must undergo recombination themselves Bacteriophages or phages are viruses that can infect a host bacterium by injecting their DNA Bacteriophage T4 belongs to a group of related bacterial viruses called T even viruses 4 DNA is contained
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