specificity. Thus there is no doubl that DNA is the direct template for its own formation.

Experimental Evidence Favors Strand Separation During DNA Replication

Simultaneously with Kornberg's research, in 1958 Matlhew Meselson and Frank VV. Stahl, then at the California Institute of Technology, carried nut an elegant experiment in which they separated daughter DNA molecules, and in so doing, showed that the two strands of the double helix permanently separate from each other during DNA replication (Figure 2-9). Their success was due in part to the use of the heavy isotope ir,N as a tag to differentially label the parental and daughter DNA strands. Bacteria grown in a medium containing the heavy isotope 15N have denser DNA than bacteria grown uuder normal conditions with HN. Also contributing to the success of the experiment was the development of procedures for separating heavy from light DNA in density gradients of heavy salts like cesium chloride. When high centrifugal Forces are applied, the solution becomes more dense at the bottom of the centrifuge tube {which, when spinning. is the farthest from the axis of rotation). When the correct initial solution density is chosen, the individual DNA molecules will move to the central region of the centrifuge tube where their density equals that of the salt solution. In this situation, the heavy molecules will form a band at a higher density (closer to the bottom of the tube) than the lighter molecules. If bacteria containing heavy DNA are transferred to a light medium (containing 14N) and allowed to grow, the precursor nucleotides available for use in DNA synthesis will be light; hence, DNA synthesized after transfer will be distinguishable from DNA made before transfer.

If DNA replication involves strand separation, definite predictions can be made about the density of the DNA molecules found after various growth intervals in a light medium. After one generation of growth, all the DNA molecules should contain one heavy strand and one light strand and thus be of intermediate hybrid density. This result is exactly what Meselson and Stahl observed. Likewise, after two generations of growth, half the DNA molecules were light and half hybrid, just as strand separation predicts.

bacteria growing in 15n; all DNA is heavy transfer to 14N medium continued growth in 14N medium bacteria growing in 15n; all DNA is heavy transfer to 14N medium continued growth in 14N medium

DNA isolated from the cens is mixed with CsCI solution (6W, p (density) -1 7g/ml) and placed in ultracenlrifuge
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