Experiment to Prove the Semiconservative Model of DNA by Mathew Meselson and Franklin Stahl

In 1958, Meselson and Stahl devised a model to investigate the model of DNA. 

Steps:

• They grew E.coli (the experimental organism) in a medium containing ¹⁵NH₄Cl (¹⁵N is the heavy isotope of Nitrogen ¹⁴N). They cultured E.coli for several generations till all the bases in the nucleotides would take up ¹⁵N
• This heavy molecule of DNA could be distinguished from the normal DNA containing ¹⁴N by centrifugation in a Cesium Chloride (CsCl) density gradient. 
  
  
• The cells containing DNA labelled ¹⁵N were transferred to a culture medium containing the normal isotope of nitrogen (¹⁴N) and allowed to grow to the generation time for E.coli (50 min at 36⁰ Celsius).

Q.  How could the two scientists distinguished the two DNA copies labelled with N15 and N14 isotopes?

A. It's not possible to distinguish the two isotopes simply by viewing them, but they can be distinguished based on their weight. N15 is heavier than N15 and that's why it looks denser. The separation is done in a centrifuge machine. 

 
The following results were found.

 

  In III generation, the ratio were: Hybrid 25% and Light 75%.

In IV generation, the ratio were Hybrid 12.5% and Light 87.5%

Note: We can see that in every generation, the hybrid percentage is decreasing and the light percentage is increasing.

Q. Can anyone answer, what would be the ratio of hybrid:light in III generation if the model is considered to be conservative?

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