What was the process of the Meelson and Stahl experiment in proving the process of DNA replication?

• The double helix structure of DNA was already proven by X-Ray diffraction, but the process of replication was still unknown

• There were 3 possible mechanisms: conservative, semi-conservative and dispersive

PROCESS

• The medium of E. Coli bacteria was used - they knew the replication time to be 20mins

-They grew a sample of E.Coli for many replications in an N-15 isotope medium so it would take it up when they replicate (refer to idea of nitrogenous bases)

• This was then transferred to an N-14 medium ONLY so that it would be incorporated into the DNA upon replication 

• They then used density gradient diffraction to separate based on density

RESULTS

• When the first sample was centrifuged there was one large band fairly far down

• Upon one replication after the transfer, there was one band formed higher up than the previous - this eliminated the conservative method of replication as this would form 2 bands due to 2 different densities - only semi-conservative and dispersive remain

• The second replication formed a band even higher and one at the level of the band from the first replication - this proved semi-conservative replication as one band was at the position of an N-15/14 hybrid and one only at N-14 - dispersive would not produce an only N-14 band