WORLD OF MICROBIOLOGY AND IMMUNOLOGY Meselson, Matthew Stanley375•
dence of that form of meningitis. Vaccines also exist against
Neisseria meningitidisand Streptococcus pneumoniaebacte-
ria, but these vaccines are only recommended for those people
who have particular susceptibility to those organisms, due to
certain immune deficiencies, lack of a spleen, or sickle cell
anemia.
Because N. meningitidisis known to cause epidemics of
disease, close contacts of patients with such meningitis are
treated prophylactically, often with the antibiotic Rifampin.
This measure generally prevents spread of the disease.See also Bacteria and bacterial infection; Viruses and
responses to viral infectionMESELSON, MATTHEWSTANLEY
(1930-Meselson, Matthew Stanley )
American molecular biologistMatthew Meselson, in collaboration with biologist Franklin
W. Stahl, showed experimentally that the replication of
deoxyribonucleic acid(DNA) in bacteriais semiconservative.
Semiconservative replication occurs in a double stranded
DNA molecule when the two strands are separated and a new
strand is copied from the parental strand to produce two new
double stranded DNA molecules. The new double stranded
DNA molecule is semiconservative because only one strand is
conserved from the parent; the other strand is a new copy.
(Conservative replication occurs when one offspring of a mol-
ecule contains both parent strands and the other molecule off-
spring contains newly replicated strands) The classical
experiment revealing semiconservative replication in bacteria
was central to the understanding of the living cell and to mod-
ern molecular biology.
Matthew Stanley Meselson was born May 24, 1930, in
Denver, Colorado. After graduating in 1951 with a Ph.D. in
liberal arts from the University of Chicago, he continued his
education with graduate studies at the California Institute of
Technology in the field of chemistry. Meselson graduated with
a Ph.D. in 1957, and remained at Cal Tech as a research fel-
low. He acquired the position of assistant professor of chem-
istry at Cal Tech in 1958. In 1960, Meselson moved to
Cambridge, Massachusetts to fill the position of associate pro-
fessor of natural sciences at Harvard University. In 1964, he
was awarded professor of biology, which he held until 1976.
He was appointed the title of Thomas Dudley Cabot professor
of natural sciences in 1976. From that time on, Meselson held
a concurrent appointment on the council of the Smithsonian
Institute in Washington, DC.
After graduating from the University of Chicago,
Meselson continued his education in chemistry at the
California Institute of Technology. It was during his final year
at Cal Tech that Meselson collaborated with Franklin Stahl on
the classical experiment of semiconservative replication of
DNA. Meselson and Stahl wanted to design and perform an
experiment that would show the nature of DNA replication
from parent to offspring using the bacteriophageT4 (a virusthat destroys other cells, also called a phage). The idea was to
use an isotope to mark the cells and centrifuge to separate par-
ticles that could be identified by their DNA and measure
changes in the new generations of DNA. Meselson, Stahl, and
Jerome Vinograd originally designed this technique of isolat-
ing phage samples. The phage samples isolated would contain
various amounts of the isotope based on the rate of DNA repli-
cation. The amount of isotope incorporated in the new DNA
strands, they hoped, would be large enough to determine quan-
titatively. The experiments, however, were not successful.
After further contemplation, Meselson and Stahl decided to
abandon the use of bacteriophage T4 and the isotope and use
instead the bacteria Escherichia coli(E. coli) and the heavy
nitrogen isotope 15N as the labeling substance. This time
when the same experimental steps were repeated, the analysis
showed three distinct types of bacterial DNA, two from the
original parent strands and one from the offspring. Analysis of
this offspring showed each strand of DNA came from a differ-
ent parent. Thus the theory of semiconservative replication of
DNA had been proven. With this notable start to his scientific
career Meselson embarked on another collaboration, this time
with biologists Sydney Brenner, from the Medical Research
Council’s Division of Molecular Biology in Cambridge,
England, and François Jacob from the Pasteur Institute
Laboratories in Paris, France. Together, Meselson, Brenner,
and Jacob performed a series of experiments in which they
showed that when the bacteriophage T4 enters a bacterial cell,
the phage DNA incorporates into the cellular DNA and causes
the release of messenger RNA. Messenger RNA instructs the
cell to manufacture phage proteins instead of the bacterial cell
proteins that are normally produced. These experiments led to
the discovery of the role of messenger RNA as the instructions
that the bacterial cell reads to produce the desired protein
products. These experiments also showed that the bacterial
cell could produce proteins from messenger RNA that are not
native to the cell in which it occurs.
In his own laboratory at Harvard University, Meselson
and a postdoctoral fellow, Robert Yuan, were developing and
purifying one of the first of many known restriction enzymes
commonly used in molecular biological analyses. Restriction
enzymesare developed by cultivating bacterial strains with
phages. Bacterial strains that have the ability to restrict foreign
DNA produce a protein called an enzyme that actually chews
up or degrades the foreign DNA. This enzyme is able to break
up the foreign DNA sequences into a number of small seg-
ments by breaking the double stranded DNA at particular loca-
tions. Purification of these enzymes allowed mapping of
various DNA sequences to be accomplished. The use of puri-
fied restriction enzymes became a common practice in the
field of molecular biology to map and determine contents of
many DNA sequences.
After many years working with the bacteria E. coli,
Meselson decided to investigate the fundamentals of DNA
replication and repair in other organisms. He chose to work on
the fruit fly called Drosophila melanogaster. Meselson dis-
covered that the fruit fly contained particular DNA sequences
that would be transcribed only when induced by heat shock or
stress conditions. These particular heat shock genes required awomi_M 5/7/03 7:52 AM Page 375