They performed experiments on E. coli to
prove that DNA replication is semi-conservative.
They first grew '[lie bacteria in a
medium containing (in
which is the heavy isotope
of nitrogen) for many generations.
Then they transferred the cells
into a medium with normal (in
thelighterisotops) and took the samples at various definite time intervals as
the cells multiplied.
The extracted DNAs were
centrifuged and measured to get their densities.
The DNA extracted from the culture
after one generation of transfer from then medium tomedium (i.e.,
after 20 minutes E.coli divides every 20 minutes) showed an intermediate hybrid
density, i.e., both heavy and light nitrogen, which proved the semi-conservative
nature of DNA.
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