MICROBIOLOGY

Microbial   Recombination  and Plasmids: Bacterial Conjugation:  The initial evidence for bacterial conjugation,the transfer of genetic information by direct cell to cell contact, came from an elegant experiment performed by Joshua Lederberg and Edward L. Tatum in 1946. They mixed two auxotrophic strains, incubated the culture for several hours in nutrient medium, and then plated it  on  minimal  medium. To  reduce  the  chance  that  their  results were due to simple reversion, they used double and triple auxotrophs on the assumption that two or three reversions would not often occur simultaneously. Lederberg and Tatum did not directly prove that physical contact of the cells was necessary for gene transfer. This evidence was provided by Bernard Davis (1950), who constructed a U tube consisting of two pieces of curved glass tubing fused at the base to form a U shape with a fritted glass filter between the halves. The filter allows the passage of media but not bacteria.

Microbial   Recombination  and Plasmids: Transposable Elements :  The chromosomes of bacteria, viruses, and eucaryotic cells contain pieces of DNA that move around the genome. Such movement is called transposition. DNA segments that carry the genes required  for  this  process  and  consequently  move  about  chromosomes are transposable elements or transposons.Unlike other processes that reorganize DNA, transposition does not require extensive areas of homology between the transposon and its destination site. Transposons behave somewhat like lysogenic prophages  except that they originate in one chromosomal location and can move to a different location in the same chromosome. Transposable elements differ from phages in lacking a virus life  cycle  and  from  plasmids  in  being  unable  to  reproduce  autonomously and to exist apart from the chromosome. They were first discovered in the 1940s by Barbara McClintock during her studies  on  maize  genetics  (a  discovery  that