Y24

2'| ...and strain Y24 cannot synthesize biotin, Phe or Cys...

ft .. .and so neither auxotrophic strain can grow on minimal medium.

2'| ...and strain Y24 cannot synthesize biotin, Phe or Cys...

.. .because genetic recombination has taken place and bacteria can synthesize all necessary nutrients.

Conclusion: Yes, genetic exchange and recombination took place between the two mutant strains.

8.10 Lederberg and Tatum's experiment demonstrated that bacteria undergo genetic exchange.

Alone, neither Y10 nor Y24 grew on minimal medium. Strain Y10 was unable to grow, because it required threo-nine, leucine, and thiamine, which were absent in the minimal medium; strain Y24 was unable to grow, because it required biotin, phenylalanine, and cysteine, which also were absent from the minimal medium. When Lederberg and Tatum mixed the two strains, however, a few colonies did grow on the minimal medium. These prototrophic bacteria must have had genotype thr+ leu+ thi+ bio+ phe+ cys+. Where had they come from?

If mutations were responsible for the prototrophic colonies, then some colonies should also have grown on the plates containing Y10 or Y24 alone, but no bacteria grew on these plates. Multiple simultaneous mutations (thr~: thr+, leu~ : leu+, and thi~ : thi+ in strain Y10 or bio~ : bio+, phe~ : phe+, and cys~ : cys+ in strain Y24) would have been required for either strain to become prototrophic by mutation, which was very improbable. Lederberg and Tatum concluded that some type of genetic transfer and recombination had taken place:

Auxotrophic strain

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