The resulting DNA molecules have the same sequence and are divided equally into the two daughter cells.įigure 9.2 Meselson and Stahl experimented with E. The new strand will be complementary to the parental or “old” strand. As a result of this experiment, we now know that during DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. Therefore, the other two models were ruled out. These results could only be explained if DNA replicates in a semiconservative manner.
The DNA harvested from cells grown for two generations in 14N formed two bands: one DNA band was at the intermediate position between 15N and 14N, and the other corresponded to the band of 14N DNA. Some cells were allowed to grow for one more generation in 14N and spun again. This suggested either a semiconservative or dispersive mode of replication. 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. They grew the bacterium, Escherichia 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 and Franklin Stahl devised an experiment in 1958 to test which of these models correctly represents DNA replication (Figure 9.2). In the dispersive model, all resulting DNA strands have regions of double-stranded parental DNA and regions of double-stranded daughter DNA.įigure by Parker, N., et.al.
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 two competing models also suggested: conservative and dispersive, which are shown in Figure 9.1.įigure 9.1 Three Models of DNA replication. After replication in this model, each double-stranded DNA includes one parental or “old” strand and one daughter or “new” strand. 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. 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 DNA replication.
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