The two chains would separate, each would guide on its surface And he and Gunther Stent published a paper I was in Chemistry. This was around sometime. Well I thought that was pretty rotten to gang up on this poor guy I had to finish my X-ray crystallography first The second experiment Actually we were rather sluggish in writing it up Mary Delbruck, Max's wife, would bring us meals, He thought that we should write it up that way.
So that is what we did. What we did was blessed by a lot of accidents. The molecular details of DNA replication are described elsewhere, and they were not known until some time after Watson and Crick's discovery. In fact, before such details could be determined, scientists were faced with a more fundamental research concern.
Specifically, they wanted to know the overall nature of the process by which DNA replication occurs. In particular, the duo hypothesized that replication occurs in a "semiconservative" fashion. According to the semiconservative replication model, which is illustrated in Figure 1, the two original DNA strands i. Conceptually, semiconservative replication made sense in light of the double helix structural model of DNA, in particular its complementary nature and the fact that adenine always pairs with thymine and cytosine always pairs with guanine.
Looking at this model, it is easy to imagine that during replication, each strand serves as a template for the synthesis of a new strand, with complementary bases being added in the order indicated. Semiconservative replication was not the only model of DNA replication proposed during the mids, however.
In fact, two other prominent hypotheses were put also forth: conservative replication and dispersive replication. According to the conservative replication model, the entire original DNA double helix serves as a template for a new double helix, such that each round of cell division produces one daughter cell with a completely new DNA double helix and another daughter cell with a completely intact old or original DNA double helix.
On the other hand, in the dispersive replication model, the original DNA double helix breaks apart into fragments, and each fragment then serves as a template for a new DNA fragment. As a result, every cell division produces two cells with varying amounts of old and new DNA Figure 1.
When these three models were first proposed, scientists had few clues about what might be occurring at the molecular level during DNA replication. Fortunately, the models yielded different predictions about the distribution of old versus new DNA in newly divided cells, no matter what the underlying molecular mechanisms.
These predictions were as follows:. First, they grew several generations of E. Next, Meselson and Stahl transferred the E. DNA synthesized after the culture was transferred to the new growth medium was composed of 14 N as opposed to 15 N; thus, Meselson and Stahl could determine the distribution of original DNA containing 15 N and new DNA containing 14 N after replication. Because the two nitrogen species have different densities, and appear at different positions in a density gradient, they could be differentiated in E.
The distribution of original DNA and new DNA after each round of replication was consistent with a semiconservative model of replication. Is it the conservative, dispersive, or semiconservative model? To answer this question experimentally, a population of E. After several generations of growth, DNA extracted from the E. Under centrifugation, cesium chloride forms a density gradient, with heavier cesium ions occupying the bottom of the test tube, and decreasing in density from the bottom of the test tube to the top.
DNA forms a band in the cesium chloride gradient, at the cesium chloride density level that corresponds to the density of the DNA. Thus, the density of the DNA can be measured by observing its position in the cesium chloride solution. The DNA extracted from E. When E. Samples taken after additional rounds of replication appeared as two bands, as in the previous round of replication. Aware of previous studies that had relied on isotope labels as a way to differentiate between parental and progeny molecules, the scientists decided to see whether the same technique could be used to differentiate between parental and progeny DNA.
If it could, Meselson and Stahl were hopeful that they would be able to determine which prediction and replication model was correct. The duo thus began their experiment by choosing two isotopes of nitrogen—the common and lighter 14 N, and the rare and heavier 15 N so-called "heavy" nitrogen —as their labels and a technique known as cesium chloride CsCl equilibrium density gradient centrifugation as their sedimentation method.
Meselson and Stahl opted for nitrogen because it is an essential chemical component of DNA; therefore, every time a cell divides and its DNA replicates, it incorporates new N atoms into the DNA of either one or both of its two daughter cells, depending on which model was correct.
The scientists then continued their experiment by growing a culture of E. In fact, they did this for 14 bacterial generations, which was long enough to create a population of bacterial cells that contained only the heavier isotope all the original 14 N-containing cells had died by then.
Next, they changed the medium to one containing only 14 N-labeled ammonium salts as the sole nitrogen source. Just prior to the addition of 14 N and periodically thereafter, as the bacterial cells grew and replicated, Meselson and Stahl sampled DNA for use in equilibrium density gradient centrifugation to determine how much 15 N from the original or old DNA versus 14 N from the new DNA was present.
For the centrifugation procedure, they mixed the DNA samples with a solution of cesium chloride and then centrifuged the samples for enough time to allow the heavier 15 N and lighter 14 N DNA to migrate to different positions in the centrifuge tube.
Following a single round of replication, the DNA again formed a single distinct band, but the band was located in a different position along the centrifugation gradient. Specifically, it was found midway between where all the 15 N and all the 14 N DNA would have migrated—in other words, halfway between "heavy" and "light" Figure 2. Based on these findings, the scientists were immediately able to exclude the conservative model of replication as a possibility. After all, if DNA replicated conservatively, there should have been two distinct bands after a single round of replication; half of the new DNA would have migrated to the same position as it did before the culture was transferred to the 14 N-containing medium i.
That left the scientists with only two options: either DNA replicated semiconservatively, as Watson and Crick had predicted, or it replicated dispersively. To differentiate between the two, Meselson and Stahl had to let the cells divide again and then sample the DNA after a second round of replication.
After that second round of replication, the scientists found that the DNA separated into two distinct bands: one in a position where DNA containing only 14 N would be expected to migrate, and the other in a position where hybrid DNA containing half 14 N and half 15 N would be expected to migrate. The scientists continued to observe the same two bands after several subsequent rounds of replication.
These results were consistent with the semiconservative model of replication and the reality that, when DNA replicated, each new double helix was built with one old strand and one new strand. If the dispersive model were the correct model, the scientists would have continued to observe only a single band after every round of replication. Following publication of Meselson and Stahl's results, many scientists confirmed that semiconservative replication was the rule, not just in E.
To date, no one has found any evidence for either conservative or dispersive DNA replication. Scientists have found, however, that semiconservative replication can occur in different ways—for example, it may proceed in either a circular or a linear fashion, depending on chromosome shape.
In fact, in the early s, English molecular biologist John Cairns performed another remarkably elegant experiment to demonstrate that E. Specifically, Cairns grew E.
Meselson and Stahl were interested in understanding how DNA replicates. They grew E. The E. The cells were harvested and the DNA was isolated. The DNA was centrifuged at high speeds in an ultracentrifuge in a tube in which a cesium chloride density gradient had been established.
Some cells were allowed to grow for one more life cycle in 14 N and spun again. Meselson and Stahl : Meselson and Stahl experimented with E. DNA grown in 15 N red band is heavier than DNA grown in 14 N orange band and sediments to a lower level in the cesium chloride density gradient in an ultracentrifuge.
When DNA grown in 15 N is switched to media containing 14 N, after one round of cell division the DNA sediments halfway between the 15 N and 14 N levels, indicating that it now contains fifty percent 14 N and fifty percent 15 N.. In subsequent cell divisions, an increasing amount of DNA contains 14 N only. These data support the semi-conservative replication model. During the density gradient ultracentrifugation, the DNA was loaded into a gradient Meselson and Stahl used a gradient of cesium chloride salt, although other materials such as sucrose can also be used to create a gradient and spun at high speeds of 50, to 60, rpm.
In the ultracentrifuge tube, the cesium chloride salt created a density gradient, with the cesium chloride solution being more dense the farther down the tube you went.
At the point, the molecules stopped sedimenting and formed a stable band. By looking at the relative positions of bands of molecules run in the same gradients, you can determine the relative densities of different molecules.
The molecules that form the lowest bands have the highest densities. So DNA grown in 15 N had a higher density, as would be expected of a molecule with a heavier isotope of nitrogen incorporated into its nitrogenous bases. Meselson and Stahl noted that after one generation of growth in 14 N after cells had been shifted from 15 N , the DNA molecules produced only single band intermediate in position in between DNA of cells grown exclusively in 15 N and DNA of cells grown exclusively in 14 N.
This suggested either a semi-conservative or dispersive mode of replication. Conservative replication would have resulted in two bands; one representing the parental DNA still with exclusively 15 N in its nitrogenous bases and the other representing the daughter DNA with exclusively 14 N in its nitrogenous bases.
The single band actually seen indicated that all the DNA molecules contained equal amounts of both 15 N and 14 N. These results could only be explained if DNA replicates in a semi-conservative manner. Dispersive replication would have resulted in exclusively a single band in each new generation, with the band slowly moving up closer to the height of the 14 N DNA band.
Therefore, dispersive replication could also be ruled out. When two daughter DNA copies are formed, they have the identical sequences to one another and identical sequences to the original parental DNA, and the two daughter DNAs are divided equally into the two daughter cells, producing daughter cells that are genetically identical to one another and genetically identical to the parent cell.
DNA replication in eukaryotes occurs in three stages: initiation, elongation, and termination, which are aided by several enzymes. Because eukaryotic genomes are quite complex, DNA replication is a very complicated process that involves several enzymes and other proteins.
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