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Chapter 10: Problem 4

Describe the role of \(^{15} \mathrm{N}\) in the Meselson-Stahl experiment.

Short Answer

Expert verified
Answer: In the Meselson-Stahl experiment, Nitrogen-15 isotope is incorporated into bacterial DNA, which serves as a key component to track DNA replication. The heavier nitrogen isotope allows the researchers to differentiate the original DNA molecules from newly synthesized ones by comparing their densities in a cesium chloride gradient. The observed banding patterns provided information on the type of replication occurring and led to the conclusion that DNA replication is semi-conservative.

Step by step solution

01

Background on the Meselson-Stahl experiment

The Meselson-Stahl experiment, conducted by Matthew Meselson and Franklin Stahl in 1958, was designed to determine the mode of DNA replication. The three proposed models at the time were conservative, semi-conservative, and dispersive replication. The experiment utilized the heavy isotope of nitrogen, \(^{15}\mathrm{N}\), as a key component to track DNA replication.
02

Incorporation of \(^{15}\mathrm{N\) into DNA

During the Meselson-Stahl experiment, \(^{15}\mathrm{N}\) was incorporated into bacterial DNA by growing E. coli cells in a medium containing the heavier nitrogen isotope for several generations. This resulted in DNA molecules that contained the heavier nitrogen isotope in place of the more abundant \(^{14}\mathrm{N}\). These heavy DNA molecules were then used as the starting material for the experiment.
03

Transfer to \(^{14}\mathrm{N\) medium and replication

After incorporating the \(^{15}\mathrm{N}\) isotope, the bacterial cells were transferred to a medium containing the lighter isotope, \(^{14}\mathrm{N}\). The bacteria were then allowed to replicate, resulting in the formation of new DNA molecules.
04

Detecting DNA densities using cesium chloride gradients

The DNA from these cells was isolated after different numbers of replication cycles. The DNA samples were then centrifuged in a cesium chloride gradient, which allowed for the separation of DNA molecules based on their density. The \(^{15}\mathrm{N}\)-labeled DNA molecules were heavier and migrated to a different position in the gradient compared to DNA molecules that contained the lighter \(^{14}\mathrm{N}\) isotope.
05

Observing replication patterns

The position of bands in the cesium chloride gradient was observed, and each band represented DNA with a specific density. The banding pattern corresponded to different DNA densities, which in turn provided information on the type of replication occurring (conservative, semi-conservative, or dispersive).
06

Conclusion

The role of \(^{15}\mathrm{N}\) in the Meselson-Stahl experiment was to allow researchers to track the DNA molecules throughout the replication process. Based on the observed banding patterns, Meselson and Stahl concluded that DNA replication is semi-conservative, where each new DNA molecule is made up of one original parent strand and one newly synthesized strand. This supported the Watson and Crick model of DNA structure proposed in 1953.

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Most popular questions from this chapter

Many of the gene products involved in DNA synthesis were initially defined by studying mutant \(E .\) coli strains that could not synthesize DNA. (a) The dnaE gene encodes the \(\alpha\) subunit of DNA polymerase III. What effect is expected from a mutation in this gene? How could the mutant strain be maintained? (b) The \(d n a Q\) gene encodes the \(\varepsilon\) subunit of DNA polymerase. What effect is expected from a mutation in this gene?

Several temperature-sensitive mutant strains of \(E .\) coli display the following characteristics. Predict what enzyme or function is being affected by each mutation. (a) Newly synthesized DNA contains many mismatched base pairs. (b) Okazaki fragments accumulate, and DNA synthesis is never completed. (c) No initiation occurs. (d) Synthesis is very slow. (e) Supercoiled strands remain after replication, which is never completed.

List the proteins that unwind DNA during in vivo DNA synthesis. How do they function?

Outline the current model for DNA synthesis.

In this chapter, we focused on how DNA is replicated and synthesized. In particular, we elucidated the general mechanism of replication and described how DNA is synthesized when it is copied. Based on your study of these topics, answer the following fundamental questions: (a) What is the experimental basis for concluding that DNA replicates semiconservatively in both bacteria and eukaryotes? (b) How was it demonstrated that DNA synthesis occurs under the direction of DNA polymerase III and not polymerase I? (c) How do we know that in vivo DNA synthesis occurs in the \(5^{\prime}\) to \(3^{\prime}\) direction? (d) How do we know that DNA synthesis is discontinuous on one of the two template strands? (e) What observations reveal that a "telomere problem" exists during eukaryotic DNA replication, and how did we learn of the solution to this problem?
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