Meselson and Stahl


The Meselson- Stahl experiment was established in 1958.

Matthew Meselson

Matthew Meselson was born on May 24, 1930 in Denver, Colorado. Meselson is known for his discovery of messenger RNA and semiconservative DNA replication. He got his Ph. D under the name of Linus Pauling at the California Institute of Technology. Then he became a professor at Harvard University in 1960. 

Franklin Stahl

Franklin W. Stahl was born in October 8, 1929 in Boton, Massachusetts. Stahl is mostly known for the Meselson - Stahl experiment. He is a professor of Biology at the University of Oregon's institute of Molecular Biology in Eugene Oregon.


Studies of bacterial transformation and bacteriaphage infections show that DNA can transmit hereditary information make control its own replication. This mechanism of DNA replication is different depending on the distribution of progeny molecules of atoms that came from their parent molecule.

Studies of bacterial transformation and bacteriaphage infection 
strongly indicate that deoxyribonucleic acid (DNA) can carry and transmit heredi
tary information and can direct its own replication. Hypotheses for the mechanism 
of DNA replication differ in the predictions they make concerning the distribution 
among progeny molecules of atoms derived from parental molecules.


     There are two separate steps to the method. The first step is known as the Density- Gradient Centrifugation. 
A small amount of DNA in a concentrated on a solution of cesium chloride and it is centrifuged until desired equilibrium is achieved. Sedimentation and diffusion together produce a stable concentration gradient for the cesium chloride. The concentration results in a increase of density toward the direction of "centrifugal force." The macromolecules of DNA more toward the section where the density matches its own density. Each "density species" DNA will form a band at its section of same density. This way we can know the DNA with N15 and from the unlabeled DNA, which will be useful in the next part of the methods. 
     The 2nd step, is the Experimental section. We get Escherichia coli B or E.coli and grow it in 360 Celsius with aeration in a glucose salts medium. We will use aeration as the nitrogen source. We then make a data of the number of colonies and cells. 
Then the e.coli with N15 were then grown for 14 generations in a medium with 
100 ,ug/ml of N'5H4Cl 
of 96.5 per cent.isotopic purity.
 During subsequent growth, the container was given fresh N14 medium containing ribosomes. Samples with 4 X 10^9 bacteria were removed before they added N14. 
     Each sample was chilled, and centrifuged in the cold for 5 minutes at 1,800 X g. The cells were lysed by adding 0.1 ml of 15 per cent sodium dodecyl sulfate and stored in the cold. "
For density-gradient centrifugation, 0.010 mnl. of the dodecyl sulfate lysate 
was added to 0.70 ml. of CsCl solution buffered at pH 8.5 with 0.01 M tris(hydroxy-
methyl)aminomethane. The density of the resulting solution was 1.71 gm. cm.-3."
     The samples were finally centrifuged in a Spinco model 
 E ultracentri
fuge at 250 for 20 hours. DNA has attained sedimentation equilibrium and the bands were found in density regions of 1.71 separated from areas of other macromolecular component of bacterial lysate. 
Pictures were taken with ultraviolet absorption.


     It takes one whole generation after adding nitrogen to the DNA for there to be only DNA with half-labeled, or unlabeled DNA. Then in the 2nd generation, the number of half-labeled and unlabeled are equal. Taking pictures, we see the transfer of nitrogen on the DNA down the generations. 
     The buoyant density of the DNA will be expected to vary because of the fraction of the N15 it was given. The outside density is only on either being labeled or unlabeled. Therefore, the degree may determine where the DNA stands between the 2 sections of fully labeled or unlabeled. 


     Watson and Crick's proposed DNA structure brought forth questions of how the molecule might replicate. These results show just how the DNA duplicates focused on the area of distribution of parental atoms among progeny molecules. By using density-gradient centrifugation, we can observe where the N15 distributes among the bacteria DNA, then following it through the transfer to its daughter molecules. The nitrogen is divided equally among the daughter DNA molecules, then those daughter molecules separate carry the nitrogen down and down. 
     The results draw a final conclusion to:
  1. The nitrogen that was attached on the DNA is equally divided into the 2 daughter DNA and it will remain intact through many generations
  2. If we follow the replication,we can see that each daughter DNA gets one half (section) of the parental sub-units
  3. The replication act results in a molecular doubling.
     The results of this experiment matches just along with Watson and Crick's model of the DNA. 
    It has been noticed that DNA from the e.coli differ from the purified salmon-sperm DNA of behavior depending on heat denaturation or exposure to high temperatures will collapse or break the rigid DNA molecule and make titration to be blocked by hydrogen-bond formation. The density can increase with no apparent molecular weight change. Heating the hybrid molecule will cause the dissociation of the NI5-containing subunit from the N14 subunit.

How Did the Experiment Change Our Knowledge of Genetics

This experiment helped conclude correctly of Watson and Crick's DNA model. People did not believe in Watson's and Crick's DNA model because they failed to see how DNA might replicate. This experiment shows and provides evidence on how DNA might do so. And it shows how DNA replication occurs correctly. During DNA replication, each new daughter DNA gets one half from the parent DNA. 

Work Cited

"Franklin Stahl." Wikipedia. Wikimedia Foundation, n.d. Web. 22 Jan. 2016.
"Matthew Meselson." Wikipedia. Wikimedia Foundation, n.d. Web. 22 Jan. 2016.
"PNAS-1958-Meselson-671-82.pdf." Google Docs. N.p., n.d. Web. 22 Jan. 2016.