Biochemistry
I N PE R S PE C T IV E
The Meselson–Stahl Experiment
DNA replication occurs before every cell division. The mechanism by which DNA
copies are synthesized is similar in all living organisms. After the two strands have
separated, each serves as a template for the synthesis of a complementary strand.
(In other words, each of the two new DNA molecules contains one old strand and
one new strand.) This process, referred to as semiconservative replication, was first
demonstrated in an elegant experiment reported in 1958 by Matthew Meselson and
Franklin Stahl (Figure 1).
In this classic work, Meselson and Stahl took advantage of the increase in
density of DNA labeled with the heavy nitrogen isotope 15N (the most abundant
nitrogen isotope is 14N). After E. coli cells were grown for 14 generations in growth
media whose nitrogen source consisted only of 15NH4Cl, the 15N-containing cells
were transferred to growth media containing the 14N isotope. At the end of both one
and two cell divisions, samples were removed. The DNA in each of these samples
was isolated and analyzed by CsCl density gradient centrifugation. (Refer to
Biochemical Methods 2.1 for a description of density gradient centrifugation.)
Because pure 15N-DNA and 14N-DNA produce characteristic bands in centrifuged
CsCl tubes, this analytical method discriminates between DNA molecules containing
large amounts of the two nitrogen isotopes. When the DNA isolated from 15N-
containing cells grown in 14N medium for precisely one generation was centrifuged,
only one band was observed. Because this band occurred halfway between where
15N-DNA and 14N-DNA bands would normally appear, it seemed reasonable to
assume that the new DNA was a hybrid molecule, that is, it contained one 15N strand
and one 14N strand. (Any other means of replication would create more than one
band.) After two cell divisions, extracted DNA was resolved into two discrete bands
of equal intensity, one made up of 14N,14N-DNA (light DNA) and one made up of
hybrid molecules (14N,15N-DNA), a result that also supported the semiconservative
model of DNA synthesis.
