By: Jessica Labrador, Adriana Martin & Rebecca Palacios
Meselson and Stahl were two scientists from the 20th century ,
who were interested in trying to find a way to prove or
disapprove Watson and cricks idea of semi-conservative
replication (Which is the normal process of DNA synthesis, in
which the two original strands of the molecule separate, and
each acts as a template on which a new complementary strand
is laid down.)

Matthew Stanley Meselson born May 24, 1930.

Franklin William Stahl born on October 6, 1929.

Franklin W. Stahl, in alliance with Matthew Meselson,
discovered evidence for the semi-conservative nature
of deoxyribonucleic acid (DNA) replication in bacteria.

In experiments, Stahl and Meselson showed that when
a double stranded DNA molecule is duplicated, the
double strands are separated and a new strand is
copied from each "parent" strand forming two new
double stranded DNA molecules.

The new double stranded DNA molecules contain one
"parent" strand and one new "daughter" strand. The
replication of a DNA molecule is semi-conservative: it
retains some of the original material while creating
some new material.

The understanding of the semi-conservative nature of
DNA in replication was a major advancement in the
field of molecular biology.
Three alternative models
of DNA replication:
A.
Conservative model- the
two parental strands
free-associate after
acting as templates for
new strands, thus
restoring the parental
double helix.
B.
Semi-conservative
model- the two strands
of the parental molecule
separate, and each
functions as a template
for synthesis of new.
Complementary strand.
C.
Dispersive model- each
strand of both daughter
molecules contains a
mixture of old and newly
synthesized DNA.
Visual
Matthew Meselson and Franklin stahl cultured E. coli
bacteria for several generations on a medium containing nucleotide
precursors labeled with a heavy isotope of nitrogen , 15n. The bacteria
incorporated the heavy nitrogen into their a DNA. The scientists
then transferred the bacteria to a medium with only 14N, the lighter,
more common isotope of nitrogen. Any new DNA that the bacteria
synthesized would be lighter than the parental DNA made in the 15N
medium. Meselson and stahl could distinguish DNA of different
densities by centrifuging DNA extracted from the bacteria.
The bands in these two centrifuge tubes represent the results
of centrifuging two DNA samples from the flask , in step 2, one
sample taken after 20 minutes and one after 40 minutes.
Meselson and stahl concluded that DNA replication follows
the semi-conservative model by comparing their results to
the results predicted by each of the three models in 16.10.
the first replication in the 14N medium produced a band of
hybrid (15N-14N) DNA. This result eliminated the
conservative model. A second replication produced both
light and hybrid DNA, a result that eliminated the
dispersive model and supported the semi-conservative
model.

Meselson and Stahl faced a tangled problem. The
Watson and Crick double helix seemed to suggest that
the two strands were completely different, each giving
rise to a new, complementary strand. The two daughter
molecules would thus contain one strand each from
the parent molecule, in a semi-conservative replication
fashion. If replication were conservative, the twisted
strands would be replicated as a whole. This would
produce one daughter molecule with all original
information and one with all new information. The
third model, termed dispersive replication, suggested
that each strand of the daughter molecule could
consist of DNA that had been mixed around so each
strand was a hybrid of old and new.

According to Meselson, “There were 2 years of things that
didn't work” followed by a year of successful experiments.
Despite the long hours, results were not immediately
forthcoming. With determination, Meselson and Stahl finally
designed a successful experiment that would help distinguish
new daughter strands from the parent strand.
Matthew Meselson
Franklin Stahl
1. The Meselson - Stahl Experiment is said to be "the most
beautiful experiment in biology.”
2. The Meselson-Stahl experiment by Matthew Meselson and
Franklin Stahl showed (finally proved) that DNA
replication was semi conservative.
3. Stahl met Meselson while partying. I quote; During his
third year of graduate school at the University of
Rochester, one of Stahl's advisors suggested that he take
a physiology course and sent him to a Marine Biological
Laboratory in Woods Hole. “I partied my way through that
course,” Stahl confesses. “During the partying, I met
Meselson,” who was also temporarily at Woods Hole,
working as a teaching assistant.
4. Both Meselson and Stahl are in their 80’s today!