questions How did semiconservative and Stahl s experiments s

questions. How did semiconservative? and Stahl s experiments support the idea that DNA replication is 2. A new form of DNA is discovered that appears to be able to replicate itself both in the 3\' 5\' direct and in the sequence direction. If this is true, how would newly on replication differ from DNA replication as we this discovered DNA know it? 3. Amazingly, an alien species of cellular organism is found alive in the remains of a meteorite that landed in the Mojave Desert. As a scientist, you are trying to determine whether this alien life-form uses DNA, protein, or some other type of compound as its hereditary material a. What kinds of experiments would you propose to determine what the hereditary material b. If the hereditary material tums out to be similar to our DNA, describe the simplest experiments you could run to try to determine if is double-stranded like our DNA, triple-stranded, or some other configuration. Copyright 2014 Pearson Activity 15.3 Inc.

Solution

Answer to question 1

Nitrogen is one of the major constituent of DNA. 14N is by far the most abundant isotope of nitrogen, but DNA with the heavier 15N isotope is also functional.
E. coli was grown for several generations in a medium containing NH4Cl with 15N. When DNA is extracted from these cells and centrifuged on a salt density gradient, the DNA separates out at the point at which its density equals that of the salt solution. The DNA of the cells grown in 15N medium had a higher density than cells grown in normal 14N medium. After that, E. coli cells with only 15N in their DNA were transferred to a 14N medium and were allowed to divide.

DNA was extracted periodically and was compared to pure 14N DNA and 15N DNA. After one replication, the DNA was found to have intermediate density. Since conservative replication would result in equal amounts of DNA of the higher and lower densities (but no DNA of an intermediate density), conservative replication was excluded. However, this result was consistent with both semiconservative and dispersive replication. Semiconservative replication would result in double-stranded DNA with one strand of 15N DNA, and one of 14N DNA, while dispersive replication would result in double-stranded DNA with both strands having mixtures of 15N and 14N DNA, either of which would have appeared as DNA of an intermediate density.

Meselson–Stahl continued to sample cells as replication continued. DNA from cells after two replications had been completed was 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. This was inconsistent with dispersive replication, which would have resulted in a single density, lower than the intermediate density of the one-generation cells, but still higher than cells grown only in 14N DNA medium, as the original 15N DNA would have been split evenly among all DNA strands. The result was consistent with the semiconservative replication hypothesis.

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