1. Brief Review of Methods of Genetic Transfer

   1. "Sexduction"/Conjugation One species actively transmits its genes directly to another member of the species.
   2. Transformation Naked DNA moves from the surrounding medium into a cell and is incorporated.
   3. Transduction Viruses are the vector as they move genes from cell to cell.

2. Lysogeny Revisited

   1. Requirement for a dependent virus What would happen were it an autonomous virus?
   2. Integrative strategy
      1. Integration
         

         LEGEND att: an E.coli seqence for the "attachment" or integration of lambda's circular chromosome. oriC: E.coli's origin of Chromosome replication (given here for orientation only) gal: E.coli's gene for galactose utilization pe:prophage ends (site of integration) cos: joined sticky ends of vegetative DNA; sometimes called ve ("vegetative ends") int: gene for the enzyme integrase c: gene for lambda repressor to maintain lysogeny Q: another gene concerned with lysogeny h: the last of the many capsomer genes.

      2. Normal Excision
         
         

   3. Non-Integrative Strategies

3. Transduction

   1. Specialized Transduction The transduced gene(s) are always the same ones.
      1. Unique att
      2. Excision accidents a la Allen Campbell (Stanford U)
         
         

      3. Defective virions (a contradiction of terms?) In exchange for picking up a few host genes, they must leave behind a few of their own genes. In the case of lambda, it leaves behind some of its late genes for capsomers in exchange for the galactose-operon.

      4. Infection and Integration When these infect a host cell, the most they can do is lysogenize, because they don't have some of the genes for lysis. When integrating into the host's chromosome, they provide the cell with a few genes from the former host cell. In the case of lambda-dg, a cell that was formerly incapable of metabolising galactose, can do so once lambda-dg has integrated.

      5. Lytic propagation and Helper viruses How then are phages such as lambda-dg propagated when they do not possess the enzymes necessary for a lytic phase?
         1. Inactivate the lambda repressor in the host lysogenized with the defective prophage
         2. Superinfect with a normal lambda
         3. Now both lambdas in the cell will be able to propagate. The burst will contain both normal lambdas as well as lambda-dg's

   2. Generalized Transduction Virtually any of the host's genes may to transduced, though, as we shall see, not necesssarily with all the same efficiencies.
      1. Phage P1
         1. Non-Integrative Lysogeny
            1. Plasmid-like state
            2. Maintenance problems of low copy number per cell
            3. Partitioning of prophages into daughter cells

         2. Lysis
            1. Packaging of the virions
            2. Miswrapping of the host DNA

         3. Usefulness in cotransduction measurements

      2. Phage Mu-1 (not µ-1)
         1. Integrative lysogeny
            1. Formation of mutated lysogens
            2. Requirement for both lysogeny and lytic routes
            3. Transposon-like action

         2. Virionic DNA
            1. 38 kbp ( = ? cistrons)
            2. Melted/Reannealed under electron microscope
            3. Gene probes
            4. Replication of DNA
            5. Procuring 'headfuls' of DNA
               • Fixed site first cut
               • Measured second site cut

      3. Usefulness

I want to go to the TOP OF PAGE or ESCAPE! or go back to the Virology Home Page!