The
15N
grown
cells
were
transferred
to
14N
medium
and
samples
were
withdrawn
at
intervals,
DNA
isolated
and
examined
by
density
gradient,
centrifugation
in
CsCi.
It
was
found
that,
hybrid
DNA
with
of
one
old
strand
and
a
new
strand
arise
soon
after
transfer
to
14N
medium
and
by
the
end
of
one
generation,
practically
no
15N
DNA
was
detected.
These
results
were
compatible
with
the
semiconservative
mechanism
of
DNA
replication.
The
detailed
mechanism
of
DNA
replication
has
however,
been
found
to
be
more
complex
than
expected
since
it
involves
the
replication
of
two
strands
which
have
opposite
polarity.
During
the
past
decade,
a
variety
of
purified
in
vitro
systems
have
been
developed
to
precisely
understand
the
mechanism
of
DNA
replication.
In
these
in
vitro
systems,
bacterial
extracts
have
been
used
to
support
the
replication
of
phage
DNA.
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