Fifty years ago, Matthew Meselson and Franklin Stahl – a Caltech grad student and post-doc, respectively – published an experiment in which they proved that DNA replication occurs when each strand copies itself to produce two identical daughter molecules, each a hybrid of old and new.
The now-famous experiment was a validation of the double-helix model of DNA, which had been proposed five years before by James Watson and Francis Crick, but was still being hotly debated. Brilliantly conceived and expertly executed, the Meselson-Stahl work has been called “the most beautiful experiment in biology.” Its details are a textbook example of scientific creativity and rigor marking the beginning of the era of molecular biology. The technique Meselson invented to separate DNA and other macromolecules—density-gradient centrifugation—is a mainstay in molecular biology labs today.
Meselson, now Thomas Dudley Cabot Professor of the Natural Sciences at Harvard’s Department of Molecular and Cellular Biology, was just 28 when he and Stahl conducted their experiment. “No matter what the age, the feeling at the time that you make a discovery, when you realize it and it crystallizes in your mind, is euphoric, of course,” Meselson says. “But then it’s like a good meal. It’s wonderful to remember, but you do get hungry again. And so it’s something you’d like to keep doing.”
Meselson’s encores came as he worked with the other greats of the early days of molecular biology, making one discovery after another about the fundamental processes of life. With Francois Jacob and Sydney Brenner, he used density-gradient centrifugation to demonstrate the existence of messenger RNA. Later, he showed that genetic recombination results from the breaking and rejoining of DNA molecules, discovered methyl-directed DNA mismatch repair, and isolated the first restriction enzyme. These discoveries, as well as his work as an advisor to the government and an activist against chemical and biological weapons, have brought him many honors, including the Albert Lasker Award for Special Achievement in Medical Science in 2004.
To commemorate the 50th anniversary of Meselson-Stahl, MCB conferred the first Meselson Award at its annual retreat in September. The annual prize will go to the graduate student who has performed the most beautiful experiment of the past year. At the event, Meselson will offer recollections of his early life in science, and Stahl will join the proceedings teleconference.
“I still remember when I first heard about the Meselson-Stahl experiment and I was amazed,” MCB Chair Catherine Dulac said. “It’s a lesson in the process of coming up with a meaningful approach to an important scientific problem, and it will be good for our students to hear about that.”
How They Did It
The idea for the Meselson-Stahl experiment had its roots in a slightly different problem that Meselson encountered as a graduate student. Jacques Monod gave a lecture on the induction of the beta-galactosidase enzyme activity by lactose in E.coli. It was unknown at the time whether exposure to the inducer activated a pre-existing enzyme, or if it triggered synthesis of new protein. Meselson came up with the idea to grow the E.coli in heavy water, letting the deuterium isotope of hydrogen label all the proteins. Then, by inducing the cells in regular water and measuring the density of the resulting enzyme activity, he could determine if the protein was newly-made.
That experiment was never done, but it provided the blueprint for the later DNA work, which really began when Meselson met Stahl at the Marine Biological Laboratory at Woods Hole in the summer of 1954. Meselson was a teaching assistant in James Watson’s physiology course; Stahl was a student in the course. A year later, Stahl landed at Caltech as a postdoc, and the two of them began to work together on methods for density separation of macromolecules. Meselson invented equilibrium density-gradient centrifugation using cesium chloride, a technique that enabled the DNA experiment by allowing the determination of very small density differences among large biomolecules.
For the pivotal experiment, the two men grew bacteria in a medium containing the heavy isotope of nitrogen (15N) for many generations until all the DNA was heavy, and then switched the organisms into media containing the lighter isotope, 14N. After the switch, Meselson and Stahl found that the DNA of the daughter cells had an intermediate density, suggesting that heavy parental DNA strands went one each to the daughters, paired with a newly synthesized light strand.
The inspiration for the experiment, and others that followed, Meselson says, was DNA itself. The double helix, he says, told researchers what to do. “The two complementary chains said ‘this is how I replicate.’ The sequence of nucleotides said ‘this is how I carry information.’ The tautomeric base forms said ‘this is how I mutate.’ The double helix was like a director, setting the agenda for research. We just had to go out and do it.”
An Interest in Sex
Meselson joined the Harvard faculty in 1960, where he has continued his long and successful search for answers, but also for compelling questions. In recent years, he has been considering the problem of sex, or more specifically, the reasons for sex. The fact that most asexual organisms are evolutionary dead-ends implies that sexual reproduction has an essential benefit that outweighs its costs. The dogma holds that sexual reproduction, and the recombination that occurs between homologous chromosomes during meiosis, is essential in shuffling the genetic deck and providing the variation upon which natural selection acts.
In 1989, Meselson heard about the bdelloid rotifers, a large and highly successful group of microscopic aquatic animals who reproduce without sex, and have been doing so for millions of years. All females, the bdelloids have been called an “evolutionary scandal.” To Meselson, they offered a unique chance to understand the advantages of sex.
What his work on the bdelloids revealed, however, is stranger even than a life with no sex. Meselson and his Harvard colleagues Irina Arkhipova and Eugene Gladyshev recently found that the rotifer genome is packed with foreign DNA, taken from plants, bacteria and even fungi. The work, published last May in Science, is important because it shows that the rotifers are not exactly genetic virgins—they incorporate external DNA into their own genomes to an extent that has not been seen before in any other animal.
How? The answer could lie in the rotifers’ unusual lifestyle. The bdelloid rotifer can survive a complete drying-out at any stage of life. This resistance to desiccation may be related to another unusual feature of the rotifers that Meselson and Gladyshev recently discovered: The bdelloids are also highly immune to the killing effects of radiation. One reason is that the animals are experts at DNA repair. Meselson believes, but has not proven, that desiccation causes the same kind of DNA damage as radiation. If so, it’s possible that, during dry periods, rotifer DNA is fragmented; foreign genes could slip in and eventually be incorporated by the DNA repair enzymes.
However, sitting in a puddle picking up random DNA is not sex. Sex involves the swapping of different versions of the same gene by homologous recombination. To see if that can happen in rotifers, the researchers are now attempting to demonstrate desiccation-induced homologous recombination of rotifer genes in the laboratory. They are also comparing the DNA sequences of closely related bdelloids collected from around the world. If otherwise quite different strains show up with regions of identical sequence, a situation called allele-sharing, that will be a sure sign of homologous gene transfer, and therefore, sexual reproduction.
If it turns out that the bdelloids have substituted desiccation for sexual reproduction, Meselson says, that would give strong support to the view that sexual reproduction, or at least some form of homologous recombination between individuals is essential for reproductive success.
A Question of Aging
The rotifer’s resistance to desiccation and radiation may also provide clues to human aging and disease. Protection from radiation in other organisms comes from a robust defense against reactive oxygen species, which damage DNA and proteins and have been blamed for aging and aging-related diseases including cancer and Alzheimer’s disease.
Meselson wonders if bdelloids are resistant to radiation and desiccation, too, because they have developed a super-scavenger that mops up reactive oxygen species before they can damage proteins. The next thing to do, he says, is to isolate this hypothetical scavenger, and see, “Is it some common old thing like glutathione or is it some super elixir of youth? This scavenger question for me is like a gift from the muse, because it’s yet another very interesting thing to work on.”
If there were a fountain of youth, one might be forgiven for thinking the 78-year-old Meselson is keeping it to himself. He has just returned from a stint working in the lab of his friend Miroslav Radman in Paris, is running his own lab at Harvard, conducts experiments at Woods Hole, and is in the middle of writing up a grant proposal on his bdelloid work.
Just before the MCB retreat, Meselson travelled to Bath, England, to accept yet another honor, the Mendel Medal from the Genetics Society of the United Kingdom. The award makes a fitting bridge from old to new: Past winners include his mentor Max Delbruck and his colleagues Jacob, Brenner, Watson and Crick. His lecture, at a special symposium on the evolution of sexual and asexual reproduction featured his current work. With a schedule like this, who’s got time for retirement?