Bacteria & Their Viruses - Chapter 8 Early on, geneticists studied eukaryotes that underwent sexual reproduction- fruit flies, corn, mice, etc. bacterial were ignored, because it was believed they did not undergo genetic transfer & had few phenotypes to compare bacteria reproduce by binary fission producing a genetically identical clone bacteria have only one copy of their chromosome so any mutation is always manifested not recessive or dominant bacteria also have plasmids independently replicating DNA circular & small disposable phenotypes are usually defined by growth conditions: -bacteria unable to utilize a nutrient for growth bacteria require minimal sources for growth some strains are unable to use certain carbon sources e.g., galactose mutant called gal- -bacteria dependent upon nutrient for growth some bacteria have lost the ability to manufacture their own nutrients from minimal media e.g. amino acids must be provided in media e.g., tryptophan is trp- known as auxotrophs wildtype counterpart is prototroph -bacteria sensitive/resistant to drug or phage e.g., antibiotics tetR or ampS -bacteria possess conditional mutations mutant phenotype under certain conditions i.e., temperature sensitivity Remember- bacterial phenotypes are changed by mutation In 1946 Lederberg & Tatum discovered genetic transfer in prokaryotes (E. coli bacteria) this happens by three different ways: conjugation transformation transduction By the way, these methods of genetic transfer in bacteria explain why the injudicious use of antibiotics can lead to the rapid spread of antibiotic-resistant bacteria. These a bilities increase the chances for bacterial survival in uncertain environments. Conjugation (Figure 8.6) requires physical contact between cells some cells (F+ cells) contain F-factor (or fertility factor) they are able to form F-pili (or sex-pili) a.k.a., conjugation tube those without F-factor cannot form pili the F-factor is located on plasmid called an F-plasmid the plasmid is transferred to the recipient cell through conjugation tube transferred as single-stranded becomes double stranded due to recipient cell's machinery recipient and donor are now both F+ some cells show high frequency of recombination (Hfr) in these cells, the plasmid has integrated into the bacterial chromosome integration occurs by recombination between plasmid & chromosome at IS elements homologous sites transposable elements F-factor (now on bact. chrom.) moves into recipient cell through conjugation tube as single- stranded DNA other genes on bact. chrom. follow! at some point the DNA being transferred breaks due to motion of bacteria new bact. chrom. DNA may be integrated into the recipient cell's genome by double recombination event if this occurs, it takes place at homologous site conjugation of Hfr cells transfers bacterial genes to recipient cells this happens at a constant rate therefore, you can determine distances between genes on bact. chrom. by the time it takes genes to be transferred: i.e., mapping! 1 min = 1 map unit interrupted mating technique see p.221 & Fig. 8.12 this mapping used on scale of larger than 2 map units the fertility factor is almost never transferred by Hfr cells because it's the last part to get transferred over F' cells are formed by imprecise excision of an F-plasmid sometimes F-plasmids are excised from bact. chrom. occasionally an adjacent gene is taken along now the F-plasmid has a gene on it phenotype of the cell is unaffected but now conjugation leads to transfer of the F-factor AND the gene recipient cell is partially diploid a merozygote Read pages 219-221 in your text and be sure that you understand Figure 8.10, how conjugation can be used to map genes on the bacterial chromosome. Transformation the uptake of free DNA by an organism (not just bacteria) double-stranded DNA is taken up by cell converted into single-stranded DNA new ssDNA displaces the recipient's DNA old DNA is removed DNA ligase joins ends of new DNA to chrom. now a heteroduplex cell divides, creating two daughter cells with different DNA one has recipient's DNA one has new DNA transformation is useful for fine-scale mapping co-transformation of two marker genes Transduction transfer and integration of genetic material between bacteria using bacteriophage as vector life cycle and growth of bacteriophage (viruses) phage are very small depend on bacteria for replication DNA or RNA chromosome bacteriophage are very specific about which bacteria they attack T4 phage as example of lytic cycle: attach to outside of bacterium inject their genetic material bacteria now make components of virus protein coat chromosome cell then lysed phage are released carried out by virulent phage sometimes phage do not kill their hosts right away lysogenic cycle phage genome integrated into bacterial chromosome now called prophage can remain part of bact. chrom. for many cell divisions at some point, they become activated and go into the lytic phase destroying cell making many phage particles this is done by temperate phage transduction can be used for genetic mapping accidental packaging of bacterial DNA into phage particle during lysis this phage then transfers bacterial DNA into recipient cell rather than phage chromosome lysis does not occur new DNA is either lost or integrated into host DNA integration is by two crossover events so arg+ gene can be transferred into arg- cell changes phenotype in recipent cell if transduction is successful mapping closely linked bacterial genes is not easy using conjugation techniques but it is possible using transduction by examining frequency of co-transduction of closely spaced genes (see Figure 8.30)