How DNA replicates itself

After the structure of DNA was elucidated by Watson and Crick, one of the next burning questions was how is it replicated? The aforementioned individuals contributed the first hypothesis: that DNA replication is semi-conservative. That is, each strand of DNA serves as a template for a newly synthesized strand. A second was the conservative hypothesis, that the entire DNA molecule serves as a template for a new DNA molecule. And finally, the dispersive hypothesis proposed by Max Delbrück argues that a mechanism exists that would break the strand every so often and attaches a new strand to the old one. To test this, Matthew Meselson and Franklin Stahl, in an incredibly elegant experiment published in 1958, grew E. coli first with 15N then with 14N and allowed to divide. They periodically extracted the DNA and centrifuged the DNA in a cesium chloride density gradient. The results were obvious. Through cell division, half of the DNA was replaced with new DNA, favoring the Watson and Crick hypothesis that DNA replication is semi-conservative.

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