12 Jan 2023


The Meselson-Stahl Experiment

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It has already been confirmed that genetic instruction are carried by nucleic acids with the culprit having been narrowed down to DNA, RNA or protein. The instant experiments seeks to scientifically confirm that it is indeed DNA and not either RNA or Protein that carries genetic information through the separation of DNA from all the other elements from a unicellular body including RNA and Protein establish if genetic instructions will remain (Rubin, 2003)

For the experimental process used two congruent bacterial pathogens which only vary in strains: the specific strain variation was however easily observable as it involved the fatality of the mammal host it was introduced to. Streptococcus pneumoniae exists in several strains with the strain III-S being virulent and fatal and II-R pneumococci being non-virulent generally benign; definitely, the fatal capabilities of III-S are carried in its genetic material and the element that can transfer that fatal capacity that is infected with the benign II-R pneumococci is the singular and confirmed carrier of genetic material.

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The experimental procedure therefore involved the thermal killing of the fatal virulent strain type III-S of Streptococcus pneumoniae and then the saline soluble components were extracted. Chloroform precipitated out proteins, carbohydrates hydrolyzed with an enzyme and then the active portion was precipitated out by alcohol fractionation . Whatever remained was confirmed to be pure DNA, a fact which was confirmed through biochemical tests that ruled out the presence of any RNA or Protein. When this DNA extract was introduced into a mouse alongside non-virulent type II-R pneumococci, the mouse developed fatal virulent type III-S pneumonia and died. (Rubin, 2003).

DNA replication 

Initially, there were 3 models that attempted to explain DNA replication, to wit (1) The conservative replication (2) The dispersive replication that involves the replication would produce two new copies of DNA and (3) Semi-conservative replication; the separation of the two original strands which combine with two new strands to form two new sets of DNA (Hanawalt, 2004)

Meselson–Stahl experiment 

The 1958 Meselson–Stahl experiment conducted by Matthew Meselson and Franklin Stahl with the help of Mason MacDonald and Amandeep Sehmbi confirmed that the two DNA would split to form two daughter strands, each daughter strand would them merge with another strand to form the new DNA molecules in a process called Semiconservative replication. Continued tests on the original DNA and resultant DNA through the presence or absence and the combinations of the 14 th and 15 th isotopes of Nitrogen clearly confirmed the contention that DNA replication only happened through semiconservative replication (Hanawalt, 2004) . 

The semi-conservative replication 

As earlier indicated, this process begins through the separation made possible by the enzyme helicases. The two separated but complete strands are known as the leading strands, with the new strands that are generated called the lagging strands. Whereas this process of separation may create strains that may affects the DNA strands, the enzyme gyrase causes the negative supercoiling of the DNA strands which prevents this strain ( Hanawalt, 2004; Masai et al., 2010 ) . 

The process of creation of the new DNA through the combination of the separated strands with new strands is conducted by DNA polymerase. However, DNA polymerase can only operate on DNA hence the need for an initiator: the initiating is done by an RNA primer which begins formation of DNA by adding the initial combinations, about 10 in number thus paving way for the DNA polymerase to take over by creating additional strand fragments known as Okazaki fragments (Masai et al., 2010). The Okazaki fragments keep on fusing with leading strand, even as they are continuously joined together by an enzyme known as DNA ligase to form the lagging strand. Once the lagging strand is fully formed, the daughter DNA is now fully formed ( Hanawalt, 2004; Masai et al., 2010 ) . 


Hanawalt, P. C. (2004). Density matters: The semiconservative replication of DNA. Proceedings of The National Academy of Sciences of The United States of America, 101 (52), 17889-17894. doi:10.1073/pnas.0407539101

Masai, H., Matsumoto, S., Zhiying, Y., Yoshizawa-Sugata, N., & Oda, M. (2010). Eukaryotic Chromosome DNA Replication: Where, When, and How? Annual Review of Biochemistry, 79 (1), 89-130. doi:10.1146/annurev.biochem.052308.103205

Rubin, J. (2003). DNA: The search for the genetic material - Avery, MacLeod & McCarty’s experiment. Retrieved from <http://www.juliantrubin.com/bigten/dnaexperiments.html/>

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