CLASSICS

Meselson and Stahl: The art of DNA replication

n 2003, the scientific community biological aspects of the necessary ex- and Stahl finally designed a successful celebrated the 50th anniversary of periments. Meselson credits Delbru¨ck experiment that would help distinguish James Watson and Francis Crick’s with giving him the information that new daughter strands from the parent landmark 1953 paper on the struc- would change the nature of the project. strand. Iture of DNA (1). The double helix, As he thrust the Watson and Crick pa- Hanawalt’s Perspective (3) outlines whose form has become the icon of bio- pers toward the young scientist, ‘‘He the intricacies of the differential nitro- logical research, was not an instant hit said, ‘Read these and don’t come back gen (14N and 15N) labeling and subse- however. The model did not gain wide until you have,’ ’’ Meselson recalls. Up quent separation of the DNA. The acceptance until the publication of an- to that point, Meselson admits that he experiments demonstrated that Watson other paper 5 years later. had not been aware of Watson and and Crick’s model of the double helix Matthew Meselson and Franklin Crick’s work or their DNA structure could replicate itself in a concerted, Stahl’s experiments on the replication of model. semiconservative fashion, and the results DNA, published in PNAS in 1958 (2), Stahl planned to go to Caltech for his were published in PNAS after being helped cement the concept of the dou- postdoctoral work, and at Woods Hole communicated by Delbru¨ck. ble helix. Meselson, a graduate student, he and Meselson decided to collaborate and Stahl, a postdoctoral researcher, on the density label project in their The Legacy of Elegant Peaks both at the California Institute of Tech- spare time. ‘‘Caltech is a cozy commu- Now, more than 45 years later, the pa- nology (Pasadena), gave validity to a nity. It’s ruled by ideas, not by walls,’’ per is still held aloft for its clarity. model that many scientists saw as specu- says Stahl. When he arrived at Caltech, Looking back, though, Meselson says lation: how two intertwined and tangled Stahl began a bacteriophage project that the paper has ‘‘one thing I wish weren’t strands of a helix could physically code did not end well after he inadvertently there.’’ At the time, published research for the material of inheritance. The Per- switched the labels on some culture from an established scientist, Paul Doty spective by Philip Hanawalt of Stanford plates. ‘‘In the midst of this gloom and (4), seemed to show that salmon sperm University (Stanford, CA), in this issue doom, Matt came in,’’ Stahl says. Me- DNA did not come apart when heated. of PNAS (3), reviews the scientific evo- selson had finished his main research Meselson and Stahl’s research could lution of this crowning achievement and project and was ready to tackle Watson then have two implications: either Doty outlines its subsequent impact on four and Crick’s hypothesis. Thus, Stahl was incorrect or Escherichia coli DNA decades of DNA replication, recombina- changed his focus from bacteriophages actually had four strands. Hence, Me- tion, and repair research. The two men to DNA replication. selson and Stahl were cautious with behind the laborious steps in discovering their wording and used the term ‘‘sub- the semiconservative replication of Not as Simple as It Seems unit’’ instead of ‘‘strand.’’ ‘‘We were lit- DNA credit much of their success to Meselson and Stahl faced a tangled tle graduate students,’’ Meselson says. timing, hard work, and serendipity. problem. The Watson and Crick double He and Stahl were wary of contradicting helix seemed to suggest that the two an established scientist. ‘‘I wish we had A Partnership Begins strands dissociated, each giving rise to a had the courage. You should believe in During his third year of graduate school new, complementary strand. The two your convictions,’’ says Meselson. Doty’s at the University of Rochester (Roches- daughter molecules would thus contain conclusions were later found to be in- ter, NY), one of Stahl’s advisors sug- one strand each from the parent mole- correct because the instruments used gested that he take a physiology course cule, in a semiconservative replication were not sensitive enough to detect the and sent him to the Marine Biological fashion. If replication were conservative, DNA molecular weight changes. Laboratory in Woods Hole, MA. ‘‘I par- however, the intertwined strands would Stahl credits the beauty and success tied my way through that course,’’ Stahl be replicated as a whole. This would of their paper to two things. First, the confesses. ‘‘During the partying, I met produce one daughter molecule with all ‘‘delightfully clean data’’ were serendipi- Meselson,’’ who was also temporarily at original information and one with all tous. The clean data peaks they ob- Woods Hole, working as a teaching as- new information. The third model, served resulted from the DNA frag- sistant. During that summer of 1954, the termed dispersive replication, consid- menting during handling; unfragmented double helix model had been well re- ered that each strand of the daughter DNA would not have separated as ceived but was only truly accepted by an molecule could consist of DNA that had nicely. Stahl likens pipetting DNA to enthusiastic minority of scientists. ‘‘Matt been shuffled around so each strand ‘‘throwing spaghetti over Niagara Falls.’’ had the idea that one ought to be able was a hybrid of old and new. The stress of the pipetting caused tre- to use density labels to test Watson’s According to Meselson, ‘‘There were mendous shearing of the DNA, although hypothesis,’’ said Stahl. Although at 2 years of things that didn’t work’’ fol- they did not realize this at the time, nor Woods Hole, Meselson was a graduate lowed by a year of successful experi- did they realize how critical this would student with Linus Pauling at the time ments. Jan Drake, then a postdoctoral be to obtaining clean peaks. In addition, at Caltech. There, Meselson had heard researcher at Woods Hole, reflected on Meselson was a ‘‘stickler for clarity,’’ Jacques Monod speak about the nature the years he shared a rented house with said Stahl. ‘‘Every single word in that of chemical bonds and enzyme synthesis, Meselson and Stahl and recalls that they paper was discussed several times before which gave Meselson a new technique all worked the same hard hours kept by ␤ being allowed to keep its position in the idea for working with -galactosidase in many graduate students and postdoc- sentence.’’ bacterial protein synthesis and measur- toral researchers today. They would Such clean data and clear writing, in ing changes in protein density. often discuss their work over dinner be- addition to the significance of the paper To explore the project, Pauling, whose fore returning to the laboratory in the for the field of molecular biology, have work centered on x-ray crystallography, evening. Despite the long hours, results sent Meselson to another Caltech pro- were not immediately forthcoming. Yet fessor, Max Delbru¨ck, to learn about the perseverance prevailed, and Meselson © 2004 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0407540101 PNAS ͉ December 28, 2004 ͉ vol. 101 ͉ no. 52 ͉ 17895–17896 Downloaded by guest on September 23, 2021 placed Meselson and Stahl’s experiment Today, the ‘‘little graduate students’’ meeting for scientists in the field of on the pages of many a syllabus. At the stay in touch. Stahl is a professor at the DNA replication every other year. Massachusetts Institute of Technology University of Oregon (Eugene, OR), President and CEO Bruce Stillman ac- (Cambridge, MA), Professor of Biology and Meselson is a professor at Harvard knowledges that it is not a large field— Tania Baker says the experiment is part University (Cambridge, MA). As defini- the attendees can fit into a single audi- of a course required of all molecular tive as the 1958 paper may appear in its torium—but states that it is a very biology graduate students. ‘‘It is a very elegance and simplicity, its greater leg- active one. Stillman says, ‘‘Forty-five nice test of a model of replication,’’ she acy is the subsequent research it has years after Meselson and Stahl, we’ve says. ‘‘Conceptually, it’s a very impor- fostered. Cold Spring Harbor Labora- still got work to do.’’ tant technique.’’ tory (Cold Spring Harbor, NY) hosts a Tinsley H. Davis, Science Writer

1. Watson, J. D. & Crick, F. H. C. (1953) Nature 171, Acad. Sci. USA 44, 671–682. 4. Rice, S. A. & Doty, P. (1957) J. Am. Chem. Soc. 964–967. 3. Hanawalt, P. C. (2004) Proc. Natl. Acad. Sci. USA 79, 3937–3947. 2. Meselson, M. & Stahl, F. W. (1958) Proc. Natl. 101, 17889–17894.

17896 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0407540101 Davis Downloaded by guest on September 23, 2021