This is from the Meselson Stahl experiment on DNA replication, hwether it follow

Question

This is from the Meselson Stahl experiment on DNA replication, hwether it follows the conservative, semi-conservative, or dispersive model. Please answer the following question:

QUESTION: Figure 2 shows two views of the same results. What is the relationship between the UV absorption bands shown in Fig. 2a and the line with two peaks in Fig. 2b?

14 15 FIG. 2-a: The resolution of N" DNA from N" DNA by density-gradient centrifugation A mixture of N14 and N1s bacterial lysates, each containing about 108 lysed cells, was centrifuged in CsCl solution as described in the text. The photograph was taken after 24 hours of centrifugation at 44,770rpm b: A microdensitometer tracing showing the DNA distribution in the region of the two bands of Fig. 2a. The separation between the peaks corresponds to a difference in buoyant density of 0.014 gm. cm.J

Explanation / Answer

Ans: The relationship between the UV absortion band in figure 2a and two eaks in figure 2b is, that it shows the difference in densities of DNA containing N14 from the DNA containing N15.
Conservative replication would result in equal amounts of DNA of the higher and lower densities, conservative replication is excluded. However, for both semiconservative and dispersive replication the result would be a DNA of intermediate density. Semiconservative replication would result in double-stranded DNA with one strand of N15 DNA, and one of N14 DNA, while dispersive replication would result in double-stranded DNA with both strands having mixtures of 15N and 14N DNA.
For excluding dispersive replication, sampling of cells needed for more replication cycles. If the DNA from cells after two replications had been completed were found to consist of equal amounts of DNA with two different densities, one corresponding to the intermediate density of DNA of cells grown for only one division in 14N medium, the other corresponding to DNA from cells grown exclusively in 14N medium, semi conservative replication hypothesis would be proved.

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