An interview with Manuel Varela and Ann Varela about Matthew Meselson: Born in Denver Colorado—Impacted the World

Nov 23, 2020 by

“…the most beautiful experiment in biology.”

—John Cairns

Michael F. Shaughnessy

1) Matthew Meselson was born in Denver, Colorado, and began his career early with a chemistry set in his basement. When exactly was Meselson born, and what do we know about his early education?

Matthew Stanley Meselson was born in Denver, Colorado, on the 24th of May in 1930, to Hyman and Ann Meselson. He was an only child. When Meselson was two years old, the family moved to Los Angeles, California.

Meselson was interested in chemistry and physics and conducted many natural science experiments at home in the garage, or should we say his first laboratory? From an early age, Meselson was fascinated by the question of how life originated. He also wondered about how electricity was related to the energy of life. Meselson attended school during the summer breaks and accrued enough credits to graduate from high school one and a half years ahead of his schooling schedule. To his dismay, the high school he attended the required three years of physical education to earn a diploma. Therefore, at the age of 16, Meselson enrolled and registered for courses at the University of Chicago. They did not necessitate a high school diploma to attend. Meselson graduated from the University of Chicago in 1951 with a Ph. B. (Bachelor of Philosophy).

Meselson attended the California Institute of Technology (Caltech) in Pasadena and earned his Ph.D. in 1957, under the direction of Linus Pauling. His research allowed him to study the details of replication of DNA in cell division with Franklin W. Stahl in 1958. They found that the cell division was “semi-conservative.”

The title of Meselson’s doctoral thesis was “I. Equilibrium sedimentation of macromolecules in density gradients with application to the study of deoxyribonucleic acid. II. The crystal structure of N, N-dimethyl malonamide.”

2) Apparently, no discussion of Matthew Meselson would be complete without a discussion of Franklin Stahl—First, who was Franklin Stahl, and what did the two of them invent?

Franklin William Stahl was born and raised in Boston. He earned a B.A. from Harvard University in 1951 and went to the University of Rochester for his graduate studies.

Near the end of completing his Ph.D. requirements, Stahl attended a molecular biology course at Woods Hole. James Watson and Francis Crick were teaching the class, and it was here that Stahl met Matthew Meselson. According to the two scientists, during a brief recess in the course, Meselson introduced himself to Stahl. Supposedly Stahl was sitting under a big tree drinking and selling beverages of the gin and tonic type. At that time, Meselson was a graduate student at Caltech; he was interested in investigating new research methods. Stahl had the experience and the mathematical skills to help Meselson design these experiments. The two got along immediately and made plans for Stahl to do post-doctoral work at Caltech.

3) Density gradient centrifugation—why is this important in scientific research?

The density gradient centrifugation technique is a long-established method for separating cellular components. The isolation of cellular parts from each other permits their purification and, hence, a closer look at their molecular mechanisms of action. A centrifuge machine is a piece of laboratory equipment shaped like a box. It has a rotor inside, which spins around at high speeds of rotation. The density gradient centrifugation uses exceptionally high rates of rotor speed rotation. The cellular contents are spun around at many times the force of gravity. The equipment is frequently referred to as an ultracentrifuge. In the ultracentrifuge machine, the rotor contains test tubes with mixtures of cell lysate. The lysate harbors sub-cellular parts, like membranes, proteins, organelles, and, importantly, nucleic acids. The spinning rotor’s ultra-fast speeds will force sub-cellular material with relatively high densities towards the test tubes’ bottoms. Meanwhile, materials with somewhat lighter densities will tend to remain towards the tops of the centrifuge tubes.

The equilibrium rates of materials depend not only on material density but also on the material sizes, shapes, and viscosities of the solvents with which the materials are centrifuged. These factors determine the extent of the sedimentation within the materials inside the spinning test tubes. Relatively dense substances will form a pellet at the bottom of the test tube, creating sediment. On the other hand, less dense substances will tend to remain towards the top of the test tube in a section called the supernatant. Investigators can separate the different biological ingredients by extracting the supernatant or accessing the pelleted material. One may describe the degree of the material depositing to the bottoms in terms of a so-called sedimentation coefficient. Typically, these coefficient values are expressed as Svedberg (S) units, named after Dr. Theodor Svedberg. He took the chemistry Nobel in 1926 for his studies of colloidal solutions in the high-speed ultracentrifuges.

Figure 53. Density gradient centrifugation.

A useful modification of the ultracentrifugation method is to add a concentrated solution of cesium chloride (CsCl) or sugar, like sucrose, in the centrifuge test tube. The ultracentrifuge rotor is then spun at excessively high rates for a long time. The high rotation rates will fractionate the solution into a gradient of densities. See Figure 53. The test tube’s bottom will take the denser solution, leaving the less dense portions at the test tube top after ultracentrifugation. The CsCl or sugar solution will form a gradually changing density gradient, from low to high density. The solution’s density gradient forms along the test tube after the ultracentrifuge has completed the high-speed rotor rotation. Therefore, the density gradient follows the centrifuged solution, increasing from top to bottom.

When radioactive material like DNA is centrifuged in this manner for about 36 hours at 30,000 revolutions per minute (rpm), the mixture of DNA and CsCl solutions reach equilibrium and form a band. See Figure 54, which was reproduced by Meselson and Stahl’s famous 1958 PNAS paper. At time 0, before the density gradient is established, the radioactive DNA is uniformly spread throughout the centrifuge tube and appears as a smear. With continued ultrahigh centrifugation for more extended periods, the banding pattern appears more compact. After about 36 hours, a tight band at equilibrium has formed.

File:Lahuse tsentrifuugimisel DNA koondumine tasakaalulisse punkti.png

Figure 54. The concentrating of DNA at the equilibrium points during solution centrifugation.

During the ultracentrifugation procedure, great care must be taken to exactly match the weights of opposing test tubes and their contents. If the opposing test tubes’ heaviness is not precisely matched, the rotor will be improperly balanced. At ultra-high speeds of rotor rotation, an imbalanced set of test tubes will be extremely unstable and cause a catastrophic failure in the ultracentrifuge function. In early incidents, unbalanced test tubes destroyed the equipment, and legendary stories emerged where entire laboratory rooms were damaged from loose rotors. In modern times, ultracentrifuges are lined with heavy shielding to prevent unstable spinning rotors from escaping their compartments. Some low-speed centrifuges are self-balancing and are, thus, not a problem. Still, an unbalanced rotor in an ultracentrifuge can extensively damage the machines.

The investigator can add a mixture of substances, like the various components of a cellular or tissue extract, to the CsCl or sugar density gradient solutions and centrifuge the test tubes. Here, the variously dense substances will move through the CsCl or sugar solutions and settle at the locations that match the same densities along the gradient as the cell and tissue parts.

Once the centrifugation is complete, the various cellular materials will have separated into bands, aligning themselves according to their matching densities along the gradient. The sample of cell debris in the density gradient solutions can be fractionated to separate them. The fractionation process is performed by puncturing the test tube’s bottom and collecting the contents in a test tube series. Each of these test tubes in the series represents a “fraction.” These fractions contain a cellular component that corresponds to the same density as the solution had sedimented. An equilibrium in density between component and CsCl has been reached.

The density gradient centrifugation method can be used to prepare various cellular parts. The cellular components include sub-cellular elements like membranes, proteins, organelles, ribosomes, or nucleic acids. Each sub-cellular piece will have specific densities and corresponding sedimentation coefficients.

4) DNA—it seems to be repeated or replicated semi-conservatively—but why is this important? 

In virtually all living forms of life on Earth, from bacteria to humans, the copying of DNA for the next generation involves DNA synthesis that uses the semi-conservative mode for the replication. That is, DNA replication is semi-conservative. In this semi-conservative mode, the newly repeated DNA has one parent strand and one recently made strand. Thus, the parental strands are “half conserved,” and the term we use is semi-conserved. See Figure 55.


Figure 55. Models of Replication. (Figure order modified.)

The semi-conservative mode of DNA replication is essential because it allows the DNA synthesizing machinery access to its internal nitrogenous bases. The base sequences serve as a template for the synthesis of new DNA. The biosynthetic DNA-making machinery needs to read the template’s nucleotide bases to know what new nucleotides to string together when making new DNA. Without easy access to the nucleotides on the inside of the DNA molecule, it would be extraordinarily difficult to read the template sequence. Hence, without access to the template’s bases, DNA synthesis would be virtually impossible. Thus, semi-conservation during DNA synthesis facilitates the transfer of new DNA into the succeeding generations and permits each new generation of the organism the opportunity to adapt and evolve to forever-changing environmental conditions.

Let us consider more closely the semi-conservative mechanism here. During DNA synthesis, the parental DNA is replicated to form the next generation’s DNA, the so-called daughter DNA—the new DNA. In general, DNA molecules are double-stranded. Each of the two parental DNA strands serves as a sequence template for replication to make two new stands. During semi-conservative DNA replication, the two strands of the parental DNA separate. Each newly constructed strand stays associated with one of the parental strands. Thus, the next generation of double-stranded DNA contains one parent and one new strand. Because the parental strands do not stay together, the replication is called semi-conservative.

In a conservative DNA replication mode, the two parental strands stay together after replication is finished. Thus, the two strands of the newly made daughter DNA would remain together, as well, after the copying is complete. Because each of the parental and the daughter strands stay together, the replication mode is called conservative. While most living cells, from bacteria to humans, do not use conservative replication, some viruses do! For instance, the double-stranded RNA reoviruses have a specially packaged protein called RNA-dependent RNA polymerase enzyme. The viral enzyme uses one of the RNA strands as a template to make the other RNA strand. However, the two parental RNA strands stay together after their replication is over. As of this writing, only the reoviruses are known to undergo conservative nucleic acid replication.

A third DNA replication mechanism is called dispersive. In a dispersive DNA replication model, the newly made DNA contains a mixture of both parental and freshly made DNA. After replication, the DNA would include interspersed segments of original and new DNA. That is, each of the two strands of the next generation’s DNA has both parental and fresh DNA.

5) It seems important to mention that Matthew Meselson studied chemistry and then did his graduate work with Linus Pauling, a scientist that needs no introduction. How did this influence his later work?

In research and teaching, Nobel Laureate Linus Pauling was a mentor extraordinaire in Meselson’s eyes. Pauling was Meselson’s graduate thesis advisor at Caltech. Meselson took his Ph.D. under the legendary Pauling, who was famous at the time for this discovery of the alpha-helix structure for proteins. Pauling would gain notoriety for his advocacy of vitamin C to prevent diseases like the common cold, and controversially, prevent cancer.

While briefly a Caltech student before graduate school, Meselson had enrolled in Pauling’s popular Freshman Chemistry course. Meselson was inspired by Pauling to concentrate on chemistry and had spent a brief period as an undergraduate studying hemoglobin protein chemistry in Pauling’s research laboratory.

In the early 1950s, Meselson became good friends with Pauling’s son Peter and daughter Linda. The Pauling family hosted a pool party in their Sierra Madre home, and Meselson was in attendance. At this event, it was reported that Pauling recruited Meselson to enter graduate school and conduct a thesis project in the Pauling laboratory at Caltech.

Meselson learned X-ray crystallography in the Pauling laboratory and, fortuitously, density gradient analysis for nucleic acids. Pauling had been cleverly astute in learning about the structural nature of proteins. He had used the method to establish the alpha-helix structural motif, and Meselson had benefited from the same expertise.

According to Meselson, in graduate school, Pauling had strongly encouraged the so-called “proposition” system for their doctoral oral examinations. The scheme involved graduate students proposing at least ten different projects and systematically defending each one. The plan was designed to fulfill Caltech’s mantra of originality, interest breadth, and proper training. Meselson later wrote that Pauling’s system ensured that the graduate student came away with a wide-ranging way of thinking outside of their expertise.

One of Meselson’s graduate committee members was the “curious character” Dr. Richard P. Feynman, Nobel Laureate and noted genius. On the day of Meselson’s Ph.D. thesis defense, May 23, 1957, Feynman commented during the question and answering session. The event quickly turned into a spectacle. Feynman had reported informed everyone in the Crellin Conference Room that he had a better method for calculating the time-course distribution of large molecules in a density gradient. Feynman had gone to the blackboard and proceeded to derive the necessary equations for support of his contention. It was an impressive performance to all in attendance, even Pauling.

Meselson successfully defended his graduate Ph.D. thesis that day. After he was informed that he passed the examination, Pauling had further informed Meselson that he was quite fortunate to be entering the field of molecular biology at the genesis of exciting discoveries. The insight would be incredibly accurate in the study of protein and DNA structures.

Meselson would be Pauling’s last graduate student, though it is unclear why, as Pauling would live for many years afterward. However, Meselson would in the succeeding years confer with Pauling on scientific matters. Moreover, Meselson would model his scientific career after the great intellectual Pauling, who had opened up nature’s forces within the chemical bond.

Caltech employed Meselson until 1960, at which time he accepted a post at Harvard University as associate professor of biology. Meselson began working at Harvard University in 1960. Meselson’s early work at Harvard University involved collaboration in 1961 with French biologist François Jacob and South African biologist Sydney Brenner. The team found that ribosomes were responsible for the construction of proteins (messenger RNA). Further research investigation with Werner Arber led to the discovery of restriction enzymes.

6) Apparently, there is even an experiment named after Meselson and Stahl—what is that all about?

Drs. Matthew Meselson and Franklin Stahl would perform what many of us in molecular biological circles consider is the “most beautiful experiment in biology.” In their famous “beautiful” experiment of 1958, Stahl and Meselson would discover that DNA synthesis followed semi-conservative replication rules. The Meselson-Stahl experiment would change the world of molecular biology forever.

As mentioned above, the three hypotheses were that DNA replication occurred by conservative, semi-conservative, or dispersive means. In the conservative replication hypothesis, the parental strands stay together. In the semi-conservative DNA synthesis hypothesis, the correct one, the daughter DNA contains half parental and half new DNA. In the premise of the dispersive model for replication, the parental and new DNA segments are interspersed throughout the next generation’s DNA.

Stahl and Meselson took advantage of two newly made radioactive isotopes of nitrogen used to tag DNA for detection. One of these radioactive nitrogen tags, nitrogen-15 (15N), was called “heavy” as it would sediment at a higher density in the density gradient centrifugation process. The other radioactive nitrogen tag, nitrogen-14 (14N), called “light,” would sediment at lighter densities.

In a control experiment performed by Meselson and Stahl, they mixed two radioactive light and heavy DNA batches. They underwent a long-term and high-speed centrifugation process. Afterward, they observed two radioactive bands (see Figure 56 left). The tracing on the left indicated the light 14N-DNA, and the tracing on the right was the heavy 15N-DNA. In Figure 56, right, a densitometer measurement of the CsCl density revealed peaks that corresponded to those of the two DNA samples, light and heavy. The banding pattern indicated an equilibrium between the DNA mixture and CsCl had been reached within the density gradient.

File:Erinevate N-isotoopidega DNA-d on erineva tihedusega. Joonisel näidatud DNA-de koondumine erinevatesse tasakaalulistesse punktidesse.png

Figure 56. DNA with different N-isotopes has different densities. The concentration of the DNAs shown in Figure at distinct equilibrium points.

Stahl and Meselson cultured Escherichia coli bacteria for many generations in heavy nitrogen (15N) in their experiment. See Figure 57, top. This first step produced bacteria with all of its parent DNA labeled with heavy nitrogen. Thus, both strands of the double helix DNA contained heavy DNA (denoted 15N-15N). Next, Stahl and Meselson broke open the bacteria, producing a cell lysate containing its DNA. They then centrifuged the radioactive DNA at a high rotor rotation rate—over 44,00 rpm for 20 hours! The result permitted the densities of the radioactive DNA and CsCl solution to reach equilibrium, producing radioactive banding patterns in their proper locations in the centrifuge tubes.

Because all of the parental DNA was heavy, it sedimented to a corresponding equally heavy density in the CsCl gradient, which was near the bottom of the centrifuge tube after the ultracentrifugation.

File:OSC Microbio 11 02 MesStahl.jpg

Figure 57. The famous Meselson-Stahl experiment of DNA replication.

Next, Stahl and Meselson took their Escherichia coli with its parental heavy-labeled DNA (15N-15N) and let it commence a “first replication” of new DNA. See Figure 57, middle. However, they switched the food source containing only light nitrogen (14N) in the medium when they permitted the bacteria to make a new DNA generation. Thus, all new DNA would have only 14N and would, therefore, be light. After one generation, the newly synthesized DNA band on the density gradient centrifugation moved towards the centrifuge tube’s middle. Stahl and Meselson quickly ruled out conservative replication because two bandings would have appeared. One would have been all heavy and the other all light because the parental DNA would have been conserved and heavy, and the new generation DNA would have been all new and light. However, after the first replication, Stahl and Meselson could not distinguish between semi-conservative and dispersive. They needed to see the second round of DNA replication. It would be one of the most exciting results ever obtained by Stahl and Meselson. See Figure 57, bottom.

After the second round of replication was finished, Stahl and Meselson cracked open the bacteria, isolated the new DNA, and ran the density gradient centrifugation experiment. This second replication result showed two bands! However, one tracing sedimented in the middle of the centrifuge tube. The other showed up towards the test tube’s top. Immediately, the presence of the two bands ruled out the dispersive model as a replication mechanism.

Instead, the two bands were interpreted by Stahl and Meselson to mean that the middle bar was a hybrid between parental and new DNA (14N-15N). The top DNA bar indicated that the second generation of DNA also had 14N-14N as its product. A dispersive mode would not have produced this hybrid in the second generation.

Let us consider the semi-conservative results more closely. In Figure 58, the parental bacteria with all heavy nitrogen, 15N, was called generation zero, produced a band at time 0 minutes. At the zero generational age, 100% of its DNA sedimented toward the bottom of the density gradient centrifuge tube. After the addition of light radioactive nitrogen, 14N, a new generation, called generation one, at 20 minutes, produced one more lightweight band, indicating a hybrid of heavy and light DNA. This indicated semi-conservative replication. When allowed to proceed to the next round of replication at generation two, 40 minutes, the top band appeared, further indicating the presence of 14N in that generation of DNA. All subsequent generations of DNA incorporated 14N, producing increasingly brighter light bands.

File:Meselson-stahl experiment diagram en.svg

Figure 58. The Meselson-Stahl experiment was an experiment by Matthew Meselson and Franklin Stahl, which demonstrated that DNA replication was semi-conservative.

The data were definitive. The best way to interpret the formation of the banding patterns in the density gradient radiograms in generations one, two, and so was that the bacteria were replicating their new DNA by the semi-conservative mechanism. Meselson and Stahl would describe the experiment in their now-classic article in the Proceedings of the National Academy of Sciences in July of 1958. It was indeed a most beautiful experiment!

7) Some of his later work involved his concern about chemical weapons in warfare—what were some of his contributions?

Meselson’s contributions have primarily involved consultation and science policy. By 1963, Meselson broadened his research to consulting for the U.S. Arms Control and Disarmament Agency concerning chemical warfare and biological defense and arms control. He advised President Richard Nixon to repudiate biological weapons production and use. Eventually, the Biological Weapons Convention was formed in 1972.

During the 1980s and 1990s, Meselson was a consultant for the CIA to investigate an anthrax outbreak in the Soviet Union as a potential biological warfare program, having suffered a military research laboratory accident in 1979. He co-authored and published the final report on the incident in the journal Science in 1994. In the article, Meselson and colleagues reported that bio-weaponized anthrax endospores were somehow accidentally released from the Sverdlovsk installation. The Bacillus anthracis endospores then dispersed through the wind and infected people and animals who lived nearby the military facility, causing the anthrax outbreak.

Meselson has also assisted as an advisor on this subject to numerous government organizations. Meselson was a member of the Arms Control and Non-Proliferation Advisory Board, which reported directly to the U.S. Secretary of State. Meselson also served as a member of the Committee on the International Security and Arms Control, which was affiliated with the National Academy of Sciences, in the U.S. Meselson has been a member of the Steering Committee as part of the Pugwash Study Group for policy determination regarding the influential group called the Implementation of the Conventions for Chemical and Biological Weapons.

8) Is he still alive and doing research, and if so, on what topics?

As we write this chapter, Meselson is 90 years old. He has received several prestigious accolades, such as the MacArthur Fellows Program Genius Award and a Guggenheim Fellowship, in addition, the Thomas Hunt Morgan Medal for a lifetime of scientific achievements, which is an award funded by the Genetics Society of America. In 2004 Meselson received the Lasker Award for Special Achievement in the Medical Sciences, and in 2008 he received the Mendel Medal, sponsored by the Genetics Society, in the U.K.

In more recent times, Meselson has been conducting investigations into the so-called Meselson effect. The phenomenon originated in the early 1990s with William Birky, who postulated the process of allelic divergence. In the process, two alleles of a gene of an individual evolve independently of each other as time goes by. Several mechanisms have been utilized to explain the Meselson effect, but some have been sources of contention.

As a young Harvard post-doctoral fellow, one of us (MFV) had the delightful opportunity to hear a keynote address delivered by Meselson at the General Meeting sponsored by the American Society for Microbiology (ASM). At the ASM meeting, I sat at the auditorium front, and the venue had thousands of other microbiologists in attendance. As I listened to the words of a famous founder of molecular biology, I was astonished by how young Meselson appeared. It is a stark reminder that the field of molecular biology is still relatively new. Afterward, Meselson’s ASM address, I followed the crowd of attendees to an even larger room filled with stations of great (and free!) food and drink. It was an inspiring evening.

For more information regarding this pioneering molecular biologist in his own words, click on the links:

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