Viruses are important to the history of molecular biologyViruses can mediate gene transfer between bacteriaBiology 212 General Genetics Spring 2007Lecture 17: Bacterial Genetics III: TransductionReading: Chap. 7 pp. 259-264; 267-274Lecture outline:1. Lytic cycle of bacterial viruses2. Generalized transduction3. Lysogenic viruses4. Specialized transductionLecture:1. Lytic cycle of bacterial viruses- Process in which the virus infects the host cell, reproduces, then lyses the cell,releasing many progeny viruses.Examples of lytic viruses: T2, T4, P1; lambda has both lytic and lysogenic cycles1) Virus attaches to bacteria and injects its DNA2) Bacterial enzymes are borrowed to reproduce the phage DNA, making multiple copies3) Bacterial DNA is broken down4) Viral genes are transcribed into mRNAs5) Viral mRNAs are translated into viral proteins6) New virus particles are assembled from DNA and proteins7) Virus causes cell lysis, releasing many progeny virusesViruses are important to the history of molecular biology- Genetic information to make progeny viruses is determined by its DNA (Hershey Chase Experiment)Viruses can mediate gene transfer between bacteria2. Generalized transduction = Transfer of bacterial DNA from one cell to another mediated by a virus.- In generalized transduction, any gene can be transferred.- Process of generalized transduction is carried out be some lytic viruses, forexample, P1 virus.Fig. 7.161) Virus DNA enters bacteria2) Viral DNA is replicated3) Bacterial chromosome is fragmented14) Viral genes are expressed5) As new viral particles are assembled, some accidently take up bacterial DNA.6) Transfer of genes from one bacteria to another can be determined by selective platingVirus infects leu+ donor bacteriumSome progeny viruses contain leu+ geneIntroduce into leu- recipient bacteriumNon-recombinants leu- Recombinants leu+Won’t grow on minimal medium Grow on minimal medium3. Lysogenic viruses Some viruses can also undergo a lysogenic cycle in which the virus integrates and sits quietly in the bacterial chromosome.Example: lambda virusFig. 7.221) Lambda virus infects the host bacteria2) Lambda virus circularizes using cohesive ends (cos sites)3) Lambda integrates into the bacterial chromosome. Similar regions (att sites) mediate pairing.4) When lambda is integrated into the bacterial chromosome, it gets replicated along with the bacterial DNA5) The lambda genes are not expressed while the virus is integrated due to action of the phage repressor protein (cI)4. Specialized transduction: Transfer of only a select set of genes mediated by a lysogenic virus.- Specialized transducing phage are produced by imprecise excision of the lambda genome from the bacterial chromosome.- Transducing particles usually pick up only a neighboring gene, either gal or