STAHL AND MESELSON’S WORK ON
SEMICONSERVATIVE(半保留) NATURE OF
DNA REPLICATION
MATTHEW STANLEY
MESELSON (1930-)
Born in Denver, Colorado
Studied chemistry at the
University of Chicago
Worked at the California
Institute of Technology
FRANKLIN WILLIAM
STAHL (1929-)
Born in Boston.
Received a B.A. from
Harvard University in
1951.
Work in the University
of Rochester after
graduated .
In 1954, Meselson met Franklin
Stahl
In 1958, they proved that DNA
was replicated in a semiconservative way.
The Meselson-Stahl experiment was an experiment
to prove that DNA replication was semiconservative.
Semiconservative replication means that when the
double stranded DNA helix was replicated, each of the
two double stranded DNA helices consisted of one
strand
Nitrogen is a major constituent of DNA, the genetic
material of all cells.
It is commonly found in the 14N isotope, but it can
also be found in the heavier 15N isotope.
E. coli were grown for several generations in a medium with 15N.
The DNA of the resuling cells had a higher density (was heavier).
E. coli cells with only 15N in their DNA were put back into a 14N
medium and were allowed to divide only once.
DNA was then extracted from a cell and was compared to DNA from
14N DNA and 15N DNA.
It was found to have exactly an intermediate density. This supported
the idea of semiconservative replication.
The DNA was intermediate in density because it had an all 15N DNA
strand and an all 14N DNA strand. The all 15N strand was one of the
original strands in the original cell. The all 14N strand was a newly
synthesized strand.
DNA replication or DNA synthesis is the
process of copying a double-stranded DNA
strand, prior to cell division
The two resulting double strands are identical,
and each of them consists of one original and one
newly synthesized strand.
This is called semiconservative replication.
The process of replication consists of three
steps, initiation, replication and termination
After the helicase unwinds the DNA, single-strand binding protein
is used to hold the DNA
RNA primase is then bound to the starting DNA site.
Begin of replication, an enzyme called DNA polymerase binds (凝
固)to the RNA primase, which indicates the starting point for the
replication.
DNA polymerase can only synthesize new DNA
The DNA polymerase can only travel on one side of the original
strand without any interruption.
Since the DNA replication on the lagging strand is not continuous,
a new DNA polymerase has to be added each time as the helicase
unwinds more DNA.
The replicated DNA is fragmented.
(DNA)n + dNTP ↔ (DNA)n+1 + PPi
Semiconservative replication describes the method by which DNA is
replicated in all known cells.
Conservative replication would leave the original, template DNA
strands intact and would produce a copy composed of entirely new
DNA base pairs.
Dispersive replication would produce two copies of the DNA, both
containing a mixture of old and new DNA base pairs.
Semiconservative replication would produce two copies that each
contained one of the original strands, and one entirely new copy.
The deciphering of the structure of DNA by Watson and Crick in 1953
suggested that the semiconservative model was correct