How Friends Goes Bad Essay

Nubia Aguilar Biology 1406 MWF 7:00- 7:50 The Meselson Stahl Experiment Exist a lot of experiments in life, but I want to write about one specific because is one of the most important in biology, it is called “Meselson Stahl Experiment”. It was an experiment by Matthew Meselson and Franklin Stahl in 1958. They supported the hypothesis that DNA replication was semiconservative. Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA helices consisted of one strand coming from the original helix and one newly synthesized.

It has been called “The most beautiful experiment in biology”. The concluded three hypotheses that had been previously proposed for the method of replication of DNA. In the semiconservative hypothesis, proposed by Watson and Crick, the two strands of a DNA molecule separate during replication. Each strand then acts as a template for synthesis of a new strand. The conservative hypothesis proposed that the entire DNA molecule acted as a template for the synthesis of an entirely new one.

According to this model, histone proteins bind to the DNA, revolving the strand and exposing the nucleotide bases for hydrogen bonding. The dispersive hypothesis is exemplified by a model proposed by Max Delbruck, which attempts to solve the problem of unwinding the two strands of the double helix by a mechanism that breaks the DNA backbone every ten nucleotides or so, untwist the molecule, and ataches the old strand to the end of a newly synthesized one.

This would synthesize the DNA in short pieces alternating from one strand to the other. Each of these three models makes a different prediction about the distribution of the “old” DNA in molecules formed after replication. In the conservative hypothesis, after replication, one molecule is the entirely conserved “old” molecule, and the other is all newly synthesized DNA. The semionservative hypothesis predicts that each molecule after replication will contain one old and one new strand.

The dispersive model predicts that each strand of each new molecule will contain a mixture of old and new DNA. The results Nitrogen is a major constituent of DNA. 14N is by far the most abundant isotope of nitrogen, but DNA with the heavier (but non-radioactive) 15N isotope is also functional. E. coli were grown for several generations in a medium 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; the progress of cell division was monitored by measuring the optical density of the cell suspension. In conclusion data from generation one conflict with conservative replication hypothesis. Data from generation two conflict with dispersive replication hypothesis. Replication is semiconservative.

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