Here it goes.......Best of luck..... Lederberg and Zinder mixed the Salmonella auxotrophic strains LA-22 and LA-2 together, and when the mixture was plated on minimal medium, they recovered prototrophic cells. The LA-22 strain was unable to synthesize the amino acids phenylalanine and tryptophan (phe-trp-), and LA-2 could not synthesize the amino acids methionine and histidine (met-his-). Prototrophs (phe+trp+ met+his+) were recovered at a rate of about 1/105 (or 10-5) cells. The two auxotrophic strains were separated by a sintered glass filter, thus preventing contact between the strains while allowing them to grow in a common medium. Surprisingly, when samples were removed from both sides of the filter and plated independently on minimal medium, prototrophs were recovered, but only from the side of the tube containing LA-22 bacteria. Since LA-2 cells appeared to be the source of the new genetic information (phe+ and trp+), how that information crossed the filter from the LA-2 cells to the LA-22 cells, allowing recombination to occur, was a mystery. The unknown source was designated simply as a filterable agent (FA). Three observations were used to identify the FA: 1) The FA was produced by the LA-2 cells only when they were grown in association with LA-22 cells. If LA-2 cells were grown independently in a culture medium that was later added to LA-22 cells, recombination did not occur. Therefore, the LA-22 cells played some role in the production of FA by LA-2 cells but did so only when the two strains were sharing a common growth medium. 2) The addition of DNase, which enzymatically digests DNA, did not render the FA ineffective. Therefore, the FA is not exogenous DNA, ruling out transformation. 3) The FA could not pass across the filter of the Davis U-tube when the pore size was reduced below the size of bacteriophages. What can you conclude from the observations: a. Transformation is occuring b. Transduction is occuring c. Conjugation has occurred d. No genetic recombination has occurred.
Posted on: Thu, 13 Mar 2014 15:40:58 +0000
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