It is a truism of scientific discovery that the final žeurekaÓ moment often occurs as a result of putting together findings made by many other contributors. Such was the case with the final elucidation of the structure of DNA. One of those who laid the foundations for the ultimate discovery was the late P&S biochemist Erwin Chargaff. Some believe that researchers searching for the structure failed to find the double helix because they ignored his simple, but significant, finding.
Erwin Chargaff left Vienna in 1935 to research blood coagulation with two surgeons at P&S. After publishing a series of papers on how clots form, Chargaff spent years working on a variety of biochemical problems. Then, in 1944, he read a paper by Oswald Avery (P&S'04) of the Rockefeller Institute for Medical Research, now Rockefeller University. Avery's research showing that DNA, not protein, transmitted genetic information stimulated Chargaff and other researchers to find out how DNA worked. "I saw before me in dark contours the beginning of a grammar of biology," Dr. Chargaff said at a meeting commemorating 100 years of nucleic acid research. "Avery gave us the first text of a new language, or rather, he showed us where to look for it. I resolved to search for this text."
Chargaff stopped his research on lipids and lipoproteins and switched full-time to DNA research. At the time, Rockefeller biochemist Phoebus Levene's tetranucleotide hypothesis of DNA was still popular. Levene postulated that DNA was made up of equal amounts of four bases adenine, guanine, cytosine, and thymine but that it was organized in a way that was too simple to enable it to carry genetic information. Because of DNA's supposed simplicity, many people thought the chromosome's proteins, not its DNA, encoded the genes even after Avery's research. Chargaff believed little solid data supported the tetranucleotide hypothesis. Only two DNA preparations had ever been examined, and the techniques available at the time were not good enough to distinguish differences in base content.
Within a couple of years, two technical advances gave Chargaff the opportunity to test the tetranucleotide hypothesis. First, Chargaff and his postdocs and students were able to adapt the technique of paper chromatography which was developed a few years earlier to separate amino acids from each other to separate the four bases in DNA. Then, with the availability of the first commercial ultraviolet spectrophotometer, Chargaff's lab could precisely measure the amount of each base in a DNA sample. The results overturned the tetranucleotide hypothesis: The bases were not present in equal quantities and they varied from organism to organism. The DNA molecule wasn't so simple after all.
Chargaff also noticed that no matter where DNA came from yeast, people or salmon the number of adenine bases always equaled the number of thymine bases and the number of guanine always equaled the number of cytosine bases. He published a review of his experiments in 1950, calling the ratios later known as Chargaff's Rules "regularities."
The meaning of the rules is clear now: They reflect the way the bases pair up in the DNA molecule, adenine with thymine and cytosine with guanine. But Chargaff had no way to determine whether his regularities were meaningful, according to historian Horace Judson. "It is not easy to see how, at the time, Chargaff could have understood the significance of the equivalence rule or taken it any further; but it remains that he did not take it further," Judson reported in his book, "The Eighth Day of Creation."
The findings languished for a couple of years until Chargaff met James Watson and Francis Crick in 1952 in Cambridge. By accounts of all the parties present, the meeting was not a success. "And it was not improved by the many farcical elements that enlivened the ensuing conversation," Chargaff wrote in his 1978 book, "Heraclitean Fire." Despite his feelings, Chargaff told the two all he knew.
Francis Crick related to historian Judson that he then realized the one-to-one ratios of the bases meant that DNA could replicate by using both strands as templates. But he and Watson didn't latch onto the idea that it could also mean the bases paired to each other until the very end of their model-building process.
Months after the meeting with Chargaff, Watson worked with cardboard cutouts of the bases, trying to fit them into a helical model of DNA revealed by the X-rays of King's College competitor Rosalind Franklin. In a recent interview with filmmakers from Windfall Films and Cold Spring Harbor Laboratory, Watson recalled, "Francis kept telling me there's Chargaff's pairs; would they pair to each other? But I didn't like Chargaff, ever since I had met him a year before. I thought: I don't want to use his data in finding the structure. Boy, it was really stupid. But I couldn't help, you know, just switching around on the table to see that adenine and thymine had formed a very nice base pair and guanine and cytosine formed one identical in shape, and I thought you can build a double helix with adenine and thymine and guanine and cytosine base pairs."
Two months later, on April 25, 1953, Watson and Crick published their structure of DNA in Nature and cited Chargaff's work. "Chargaff's discovery that there was quantitative relationship of A to T and G to C was one of the main events in the DNA story, along with the discoveries of Avery and Rosalind Franklin," says Dr. Isidore Edelman, Robert W. Johnson Jr. Professor Emeritus and chairman of biochemistry at P&S from 1977 to 1988. Dr. Edelman, who knew Chargaff toward the end of his career says, "I think Chargaff and Franklin deserved a Nobel Prize for their critical contributions to the foundation of the double helix model."