Thursday, March 18, 2010


Scientists have pioneered through the field of genetics and DNA trying to learn more and understand this complex structure. Hershey and Chase, Griffith and Avery, and Meselson and Stahl conducted three of the most important experiments of DNA. These remarkable discoveries helped scientists today to understand more about DNA and hopefully use this information to their advantage. In their first experiment they labeled the DNA of phages with radioactive phosphorus-32, which is present in DNA but not present in protein. They allowed the phages to infect E.Coli bacterium, and then removed the protein shells from the infected cells with a blender and centrifuge. They saw that the radioactive phosphorus was only visible in the bacteria cells and not the protein shells. In the second part of their experiment, they labeled the phages with radioactive sulfur-35, which is present in protein but not DNA. After separating the cell from the protein shell, the radioactive sulfur was found in the protein shell, but no in the infected bacteria, confirming that DNA is actually the genetic material of the phage that infects the cell. This was a huge step for scientists in the field of genetic material, because most people believed that protein had been the genetic material of a cell, but from this experiment, since the radioactive phosphorus was found in the DNA and not the protein, and the radioactive sulfur was found in the protein but not the DNA, it is concluded that DNA is indeed the genetic material.
Frederick Griffith was a British medical officer and geneticist who conducted an experiment, in which he discovered the “transforming principle”, which is today known as DNA. This famous experiment was done when Griffith was attempting to make a vaccine to prevent pneumonia in the influenza epidemic after World War I, using two strains of Streptococcus pneumoniae bacterium. The smooth strain was injected into mice and these mice died from pneumonia in a day or two. The smooth strain had a capsule on it that allowed the bacteria to resist the immune system. The rough strain was then injected into the mice, but they did not die, because the rough strain did not have a capsule. Griffith and Avery then heated the smooth strain and it did not kill the mice. But when the dead smooth strain was mixed with the live rough strain, the mouse died. Griffith and Avery were curious about this and he isolated the bacteria from the blood of the rough/smooth strain mouse. Griffith and Avery discovered that the rough strain had obtained capsules from the smooth strain bacteria, making all the bacteria smooth strained. Griffith and Avery then hypothesized that a “transforming principle” from the heat killed smooth strain converted the rough strain into the smooth strain. According to the Griffith and Avery experiment, the hypothesis is that the E.coli cells with the ampicillin resistant gene will continue to grow when placed in an agar with ampicillin. This occurs because the ampicillin resistant gene goes through the transformation process and enters the DNA of the E. coli completely changing the cell, allowing it to continue growing when it meets the antibiotic. This provided chemical evidence for the nature of the gene, because it proved that DNA the genetic material was transferred from one cell to another, even when the strain was killed by heat.
The Meselson-Stahl experiment was an experiment by Matthew Meselson and Franklin Stahl which demonstrated that DNA replication was semiconservative. Semiconservative replication is when the double stranded DNA helix replicated and each of the two double stranded DNA helices consisted of one strand coming from the original helix and one newly synthesized. E. coli. These were grown for several generations in a medium with nitrogen-15. 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 resulting cells had a higher density (was heavier). After that, E. coli cells with only nitrogen-15 in their DNA were put back into a nitrogen-14 medium and were allowed to divide only once. DNA was then extracted from a cell and was compared to DNA from nitrogen-14 DNA and nitrogen-15 DNA. It was found to have close to the intermediate density. Since conservative replication would result in equal amounts of DNA of the higher and lower densities, conservative replication was not included . But the results were consistent with both types of replication. Semiconservative replication would result in double-stranded DNA with one strand of nitrogen-15 DNA, and one of nitrogen-14 DNA, while dispersive replication would result in double-stranded DNA with both strands having a mixture of nitrogen-14 and nitrogen-15 DNA, either of which would have appeared as DNA of an intermediate density.
DNA was then extracted from cells which had been grown for several generations in a nitrogen-15 medium, followed by two divisions in a nitrogen-14 medium. DNA from these cells was found to consist of equal amounts of two different densities, one corresponding to the intermediate density of DNA of cells grown for only one division in nitrogen-14 medium, the other corresponding to cells grown exclusively in nitrogen-14 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 nitrogen-14 DNA medium, as the original nitrogen-15 DNA would have been split evenly among all DNA strands. The result was consistent with semiconservative replication, in that half of the second-generation cells would have one strand of the original nitrogen-15 DNA along with one of nitrogen-14 DNA, accounting for the DNA of intermediate density, while the DNA in the other half of the cells would consist entirely of nitrogen-14 DNA, one synthesized in the first division, and the other in the second division.

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