In present times, humans have a vast understanding of DNA. In order to have gained this knowledge of the complex concept, it is clear many experiments have taken place. There are four in particular, in which these experiments have greatly contributed to the human comprehension of DNA: the Griffith experiment, Avery experiment, Hershey-Chase experiment, and the Meselson-Stahl experiment.
Being the oldest of the four, the Griffith experiment took place in 1928. Microbiologist Frederick Griffith discovered transformation when he tried to answer the question: “what is the cure to pneumonia?” Griffith apprehended this study by injecting mice with different strains of pneumococcus bacteria. He used type III-S and type II-R. The III-S type has a smooth polysaccharide coat/capsule, making it resistant to the immune system of mice, while the II-R type lacks this coat. The III-S strain was found to kill the mice, while live II-R left the mice living and with few symptoms. In a second round, Griffith injected mice with III-S bacteria that had been killed by heat. Since the dead bacteria had been rendered ineffective by the heat, the mice did not die. Griffith then injected a mixture of the heat-killed S bacteria and live R bacteria. Both of the bacteria types in the mixture were not expected to harm to harm the mice, but in an interesting turn of events, the mice developed diseases and most died. Interestingly, the blood of the mice contained high levels of live III-S bacteria. It was deduced that the “information specifying the polysaccharide capsule had passed from the dead…S bacteria to the live, capsuleless R bacteria” (Johnson 218). The type III-S strain transformed into the type II-R strain. The Griffith experiment is a key contributor to the human’s understanding of DNA, for the experiment in itself was the discovery of transformation. Merriam Webster defines transformation as “genetic modification of a cell by the uptake and incorporation of exogenous DNA.” Griffith’s experiment provided part of “the key evidence that DNA is the genetic material,” which is arguably the most important feature of DNA.
The next experiment is the Avery experiment, another key contributor. In 1944, Oswald Avery and his colleagues experimented in order to answer the question: “what type of substance could change the characteristics of the organism that received it?” (4.2 DNA CK-12). Avery and his coworkers created the same mixture of dead III-S and live II-R as Griffith had done years before. However, in this experiment, they removed the protein so much as to attain 99.98% purity. Despite doing so, the II-R strain continued to transform into the deadly strain, thus proving that proteins were— in fact— not the genetic material. The researchers then inactivated DNA in the S strain; the R strain remained harmless. Inevitably, the experiment evinced one main conclusion: DNA is the hereditary material. The Avery experiment is a considerable contributor to the current society’s understanding of DNA, for it is what validated the main property of DNA.
Coterminous to the previous came the Hershey-Chase experiment, which derived from the essential question: is DNA the genetic material? In order to procure the answer, Alfred Hershey and Martha Chase “studied the genes of viruses that infect bacteria” (Johnson 219). The bacteria-infecting viruses attach themselves to bacteria and inject their genes into the interior. Once inside, the virus takes over the genetic machinery. Hershey and Chase labeled DNA and protein: DNA with radioactive phosphorus, and protein coats with radioactive sulfur. These isotopes were used as tracers to indicate whether it was protein or DNA that was injected into the cell (Johnson 219). They allowed the viruses to infect the bacteria, and they then dislodged the bacteria from the surface. As expected, they found radioactive phosphorus in the interior of the cells infected by viruses containing the said isotope. In bacterial cells infected by radioactive sulfur-tagged viruses, there were no traces of radioactive sulfur. The results elucidate the unequivocal fact that “the genes that viruses use to specify new viruses are made of DNA and not protein” (Johnson 219). This experiment was allowed biologists to advance in their studies of DNA, for it indisputably proved that “DNA was the genetic material, [since] DNA was transferred to the bacteria” (Hershey and Chase). Most biologists did not believe the results of the Avery experiments, but the Hershey-Chase experiment confirmed the results.
Last is the Meselson-Stahl experiment. The experiment derived from the essential question: how does DNA replicate itself? In 1958, Matthew Meselson and Franklin Stahl distinguished parental and daughter DNA by “modifying the molecules so each kind had a different density” (Meselson-Stahl Experiment). They grew bacteria for several generations in a substance containing the heavy isotope of nitrogen, and they then transferred the bacterial cells to a new medium containing the light isotope of nitrogen. The researchers collected samples at twenty-minute intervals. Following this, they extracted DNA samples and suspended the samples in cesium chloride. Using centrifugation, they separated the DNA strands of different densities. Thanks to the centrifugal forces, the cesium ions migrated toward the bottom, “creating a gradient of cesium concentration, and thus a gradation density” (Johnson 222). Each DNA strand then reached the position where its density was equivalent to the density of the cesium in that specific location. The dense DNA collected from the test tubes showed that after the bacteria went through its first round of DNA replication in the light nitrogen isotope medium, the density of the DNA decreased to a value intermediate between light nitrogen isotope DNA and heavy nitrogen isotope DNA. After the second round of replication, there were two classes of DNA, “one intermediate and one equal to that of [light nitrogen DNA]” (Johnson 222). Meselson and Stahl came to the conclusion that “each daughter DNA molecule contains one new daughter subunit and one subunit conserved from the parental DNA molecule” (Meselson-Stahl Experiment). This experiment contributed to human knowledge of DNA by verifying the Watson-Crick model and demonstrating the semiconservative replication of DNA.
All factors considered, each and every experiment was a key benefactor to a scientist’s overall knowledge DNA. The Griffith experiment itself was the discovery of transformation. The Avery experiments and the Hershey-Chase experiment proved that DNA—not protein— is hereditary material. The Meselson-Stahl experiment aided scientists in understanding the process of semiconservative replication. In general, the topic of DNA is profoundly complex, with several aspects that one may find difficult to understand. The experiments above allow people to understand more about the history of science and DNA itself.