By: Jessica Labrador, Adriana Martin & Rebecca Palacios

Meselson and Stahl were two scientists from the 20 th century, who were interested in trying to find a way to prove or disapprove Watson and cricks idea of semi-conservative replication (Which is the normal process of DNA synthesis, in which the two original strands of the molecule separate, and each acts as a template on which a new complementary strand is laid down.)

 Matthew Stanley Meselson born May 24,  Franklin William Stahl born on October 6, 1929.

 Franklin W. Stahl, in alliance with Matthew Meselson, discovered evidence for the semi-conservative nature of deoxyribonucleic acid (DNA) replication in bacteria.  In experiments, Stahl and Meselson showed that when a double stranded DNA molecule is duplicated, the double strands are separated and a new strand is copied from each "parent" strand forming two new double stranded DNA molecules.  The new double stranded DNA molecules contain one "parent" strand and one new "daughter" strand. The replication of a DNA molecule is semi-conservative: it retains some of the original material while creating some new material.  The understanding of the semi-conservative nature of DNA in replication was a major advancement in the field of molecular biology.

Three alternative models of DNA replication: A. Conservative model- the two parental strands free-associate after acting as templates for new strands, thus restoring the parental double helix. B. Semi-conservative model- the two strands of the parental molecule separate, and each functions as a template for synthesis of new. Complementary strand. C. Dispersive model- each strand of both daughter molecules contains a mixture of old and newly synthesized DNA. Visual

Matthew Meselson and Franklin stahl cultured E. coli bacteria for several generations on a medium containing nucleotide precursors labeled with a heavy isotope of nitrogen, 15n. The bacteria incorporated the heavy nitrogen into their a DNA. The scientists then transferred the bacteria to a medium with only 14N, the lighter, more common isotope of nitrogen. Any new DNA that the bacteria synthesized would be lighter than the parental DNA made in the 15N medium. Meselson and stahl could distinguish DNA of different densities by centrifuging DNA extracted from the bacteria.

The bands in these two centrifuge tubes represent the results of centrifuging two DNA samples from the flask, in step 2, one sample taken after 20 minutes and one after 40 minutes.

Meselson and stahl concluded that DNA replication follows the semi-conservative model by comparing their results to the results predicted by each of the three models in the first replication in the 14N medium produced a band of hybrid (15N-14N) DNA. This result eliminated the conservative model. A second replication produced both light and hybrid DNA, a result that eliminated the dispersive model and supported the semi-conservative model.

 Meselson and Stahl faced a tangled problem. The Watson and Crick double helix seemed to suggest that the two strands were completely different, each giving rise to a new, complementary strand. The two daughter molecules would thus contain one strand each from the parent molecule, in a semi-conservative replication fashion. If replication were conservative, the twisted strands would be replicated as a whole. This would produce one daughter molecule with all original information and one with all new information. The third model, termed dispersive replication, suggested that each strand of the daughter molecule could consist of DNA that had been mixed around so each strand was a hybrid of old and new.

 According to Meselson, “There were 2 years of things that didn't work” followed by a year of successful experiments. Despite the long hours, results were not immediately forthcoming. With determination, Meselson and Stahl finally designed a successful experiment that would help distinguish new daughter strands from the parent strand. Matthew Meselson Franklin Stahl

1.The Meselson - Stahl Experiment is said to be "the most beautiful experiment in biology.” 2.The Meselson-Stahl experiment by Matthew Meselson and Franklin Stahl showed (finally proved) that DNA replication was semi conservative. 3.Stahl met Meselson while partying. I quote; During his third year of graduate school at the University of Rochester, one of Stahl's advisors suggested that he take a physiology course and sent him to a Marine Biological Laboratory in Woods Hole. “I partied my way through that course,” Stahl confesses. “During the partying, I met Meselson,” who was also temporarily at Woods Hole, working as a teaching assistant. 4.Both Meselson and Stahl are in their 80’s today!