Meselson and Stahl conducted an experiment using E. coli bacteria and isotopes of nitrogen to test theories of DNA replication. They grew bacteria in a medium containing a heavy isotope of nitrogen, then transferred them to a light isotope medium. When they analyzed the DNA bands produced after centrifugation, they found bands corresponding to the predicted patterns of semi-conservative replication. By heating samples, they confirmed that each strand of replicated DNA contained either all heavy or all light nucleotides. This provided definitive evidence supporting the Watson-Crick model of semi-conservative DNA replication.

1. BY RABBI C. BAIDOO
UNIVERSITY OF ENERGY
AND NATURAL
RESOURCES-SUNYANI

2. PRESENTATION OBJECTIVES
At the end, you should be able to;
• Explain DNA replication
• Describe the three models of DNA replication
• Explain how Meselson and Stahl established the basic mechanism
of DNA replication as semi- conservative.
• Describe how Meselson and Stahl showed that each strand
identified in the CsCl medium was either all heavy or all light

3. INTRODUCTION
The base complementarity of Watson - Crick Model immediately
suggested that DNA should replicate with each strand remaining intact
and specifying a new complementary strand.
This mode is called semi-conservative because each strand is
conserved –kept intact-even though the double helix is not.
Matt Meselson and Franklin Stahl originally met in the summer of
1954, the year after Watson and Crick published their paper on the
structure of DNA.

4. In 1958, Mathew Meselson and Franklin W. Stahl found a superb
way to test the prediction of the Watson - Crick Model regarding
replication.
The three models for DNA replication

5. Experiment begins
Materials: Ultracentrifuge (60,000 rev/min),
E. coli bacterial cell, NH4Cl (15N source),
Cesium chloride (density = 1.7 g/ml)
STEPS;
• The scientist grew E. coli in 15N medium
for 14 generations
• Bacteria took up the 15N and used it to
synthesize new biological molecules
including DNA.

6. Cont….
• At this time they isolated the first bacteria
sample, prepared the DNA and added the
CsCl for centrifugation.
• At the same time, some of the 15N coated
bacteria were switched to medium
containing a "light" 14N isotope and
allowed to grow for several generations.

7. cont…..
• They called the 1st sample generation 0
• After the transfer to light medium,
• A sample was taken every 20mins which is the generation time
for E coli cells to grow under optimum temperature
• The DNA from the samples were prepared for high speed
centrifugation. (60,000rev/min)

8. Cont….
• They found that the salt ions (CsCl) form a density
gradient, a solution whose density is low at the top of
the tube and increases steadily toward the bottom.
• Large molecules in the solution
will find an equilibrium position
where their buoyant density equals
that of the gradient.

9. EXPERIMENT RESULTS
When DNA from the first four generations of E. coli was analyzed, it
produced the pattern of bands shown in the figure below:

10. After the Experiment;
Meselson and Stahl then had to show that the units being separated in
the cesium chloride (CsCl) gradient were double stranded molecule
with each strand either all heavy or all light.
Their evidence for this took advantage of a useful property of DNA:
since the double helix is held together by weak hydrogen bonds, heat of
80-100 °C will separate the strand by disrupting or melting this bonds.
Meselson and Stahl melted/heated some of their half-dense DNA and
showed that is separates into one dense fraction and one light fraction
as it should be as it should.

11. They concluded that each strand of half dense
DNA consists of either totally dense or totally
light nucleotides, not a mixture of dense and light
nucleotides in one strand.
This work provided a definitive confirmation of
Watson - Crick Model.

12. CONCLUSION
The experiment done by Meselson and Stahl demonstrated that DNA
replicated semi-conservatively, meaning that each strand in a DNA
molecule serves as a template for synthesis of a new, complementary
strand.
Although Meselson and Stahl did their experiments in the bacterium
E. coli, we know today that semi-conservative DNA replication is a
universal mechanism shared by all organisms on planet Earth.
Some of our cells are replicating their DNA semi-conservatively right
now!

13. REFERENCES
1. Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., and Jackson, R. B.
(2011). The basic principle: Base pairing to a template strand. In Campbell biology (10th ed.).
San Francisco, CA: Pearson, 318-319.
2. Davis, T. H. (2004). Meselson and Stahl: The art of DNA replication. PNAS, 101(52),
17895-17896.
3. Watson, J. D., and Crick, F. H. C. (1953). Genetical implications of the structure of
deoxyribonucleic acid. Nature, 171, 740-741.
4. Davis, T. H. (2004). Meselson and Stahl: The art of DNA replication. PNAS, 101(52),
17895-17896.
5. Meselson, M. and Stahl, F. W. (1958). The replication of DNA in Escherichia coli. PNAS,
44(7), 671-682. Retrieved from
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC528642/pdf/pnas00686-0041.pdf.