04 Molecular Basis of Inheritance - part 04 - Semiconservative Replication - Experimental confirmation

04 Molecular Basis of Inheritance - part 04 - Semiconservative Replication - Experimental confirmation


Experimental confirmation : 

Semiconservative Replication : 
  • In newly formed DNA molecule, one strand is old (i.e. conserved) and other strand is newly synthesized. Thus, it is called Semiconservative mode of replication. 
  • It is experimentally proved by Matthew Meselson and Franklin Stahl (1958) by using equilibrium - density - gradient centrifugation technique. 
  1. Meselson and Stahl in 1958 performed an experiment to prove semiconservative nature (mode) of replication. 
  2. They cultured bacteria E.coli in the medium containing 14N (light nitrogen) and obtained equilibrium density gradient band by using 6M CsCl2. The position of this band is recorded.  
  3. E. coli cells were then tranferred to 15N medium (heavy isotopic nitrogen) and allowed to replicate for several generations
  4. At equilibrium point density gradient band was obtained, by using 6M CsCl2. The position of this band is recorded.
  5. The heavy DNA (15N) molecule can be distinguished from normal DNA by centrifugation in a 6M Cesium chloride (CsCl2) density gradient. 
  6. The density gradient value of 6M CsCl2 and 15N DNA is almost same. Therefore, at the equilibrium point 15N DNA will form a band. In this both the strands of DNA are labelled with 15 N. 
  7. Such E. coli cells were they transferred to another medium containing 14N i.e. normal (light) nitrogen. After first generation, the density gradient band for 14N 15N was obtained and its position was recorded. 
  8. After second generation, two density gradient bands were obtained - one at 14N 15 N position and other at 14N position. 
  9. The position of bands after two generationsclearly proved that DNA replication is Semiconservative.




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