![]() ![]() Meselson and Stahl noted that after one generation of growth in 14N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15N or 14N. DNA grown in 15N would be expected to form a band at a higher density position than that grown in 14N. ![]() The DNA was separated by ultracentrifugation, during which the DNA formed bands according to its density. The cells were harvested and the DNA was isolated. coli culture was then shifted into a medium containing 14N and allowed to grow for one generation. coli for several generations in a medium containing a “heavy” isotope of nitrogen ( 15N) that was incorporated into nitrogenous bases and, eventually, into the DNA. Matthew Meselson (1930–) and Franklin Stahl (1929–) devised an experiment in 1958 to test which of these models correctly represents DNA replication (Figure 2). In the dispersive model, all resulting DNA strands have regions of double-stranded parental DNA and regions of double-stranded daughter DNA. In the semiconservative model, parental strands separated and directed the synthesis of a daughter strand, with each resulting DNA molecule being a hybrid of a parental strand and a daughter strand. In the conservative model, parental DNA strands (blue) remained associated in one DNA molecule while new daughter strands (red) remained associated in newly formed DNA molecules. There were three models suggested for DNA replication. There were two competing models also suggested: conservative and dispersive, which are shown in Figure 1.įigure 1. ![]() In one model, semiconservative replication, the two strands of the double helix separate during DNA replication, and each strand serves as a template from which the new complementary strand is copied after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand. Separating the strands of the double helix would provide two templates for the synthesis of new complementary strands, but exactly how new DNA molecules were constructed was still unclear. The elucidation of the structure of the double helix by James Watson and Francis Crick in 1953 provided a hint as to how DNA is copied during the process of replication.
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