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Download Bacterial Genetics:Binary Fission, Transduction,Transformation
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Bacterial Genetics (Ch18) Genetic diversity caused by Rapid reproduction Mutation Recombination Bacteria – One of the simplest genetic model systems to study the mechanisms of molecular genetics Escherichia coli (E. coli) – Intestinal flora – “lab rat” of molecular biology Replication of bacterial genome • One circular DNA genome – Single origin of replication (Ori) – Bidirectional DNA replication • May have plasmids – smaller circular DNA molecules • Autonomously replicated (contain ori) • • Bacteria divide by binary fission • asexual reproduction • Progeny are genetically identical to parent (clones) LE 18-14 Bacterial DNA replication Replication fork Origin of replication Both genome & plasmid replicate in this manner Termination of replication Bacterial DNA Mutation Caused spontaneously (mistakes in DNA synthesis) – Physicochemical forces (UV, X rays, chemical mutagens, etc.) •Since reproduction is quick e.g.doubling time=20 min New mutations spread quickly Calculation of incidence of mutations If doubling time= 20 min, then 23cells/hr Over 12 hr, 236 cells (~1010)produced from a single cell If spontaneous mutation rate = 1 x 10-7 / gene, then in 12 hr (day) (1010) (10-7)= 103 mutations/gene/day If bacteria have ~4000 genes then (4x103genes)(103)= 4 x 109 mutations/day Take home message: bacterial mutations rare per gene But due to rapid cell division, become frequent Major contribution to genetic diversity and ability to adapt Another source of bacterial genetic diversity: Genetic Recombination • Three processes bring bacterial DNA from different individuals together: – Transduction – Transformation – Conjugation Transduction • Bacteriophages (bacterial viruses) transfer bacterial genes from one host cell to another LE 18-16 Phage DNA A+ B+ A+ B+ Donor cell A+ Crossing over A+ A- B– Recipient cell A+ B– Recombinant cell Transformation • Alteration of a bacterial cell’s genotype and phenotype – by the uptake of naked, foreign DNA from the surrounding environment •For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells •Uptake of ampicillin resistant/ GFP-carrying plasmid (done in lab) (pGLO) Conjugation • Direct transfer of DNA between live bacterial cells that are temporarily joined • Transfer one-way: – “Male” donates DNA – “Female” receives DNA • “Maleness,” – Contains F (fertility) genes on plasmid or in genome – Encode sex pilus • Forms passage way for DNA from donor to recipient LE 18-17 Sex pilus 5 µm • Donor cells containing the F plasmid: F+ • Recipient cells: F• Cells with F factor integrated into genome: Hfr cell (high frequency of recombination) • Hfr cells – Transfer some genomic DNA to recipient cell LE 18-18_4 F plasmid Bacterial chromosome F+cell Mating bridge F– cell F+ cell F+ cell Bacterial chromosome Conjugation and transfer of F plasmid from and F+ donor to an F– recipient Hfr cell F+ cell F factor Hfr cell Formation of Hfr (high frequency of recombination) cell F– cell Temporary Recombinant F– partial bacterium diploid Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombiination Questions???
