One of those who laid the foundations for the ultimate discovery was the late P&S biochemist Erwin Chargaff. His simple, but significant, finding on the molecules composition helped lead James Watson and Francis Crick to the double helix. The information proved so critical that other researchers searching for DNAs structure may have failed to find the double helix because they ignored Chargaffs Rules.
Erwin Chargaff was born in Austria and worked in Vienna, Berlin, and Paris before leaving Europe in 1934 for Mount Sinai Hospital in New York. In 1935 he came to P&S to research blood coagulation with two surgeons. After publishing a series of papers on how clots form, Dr. Chargaff spent years working on a variety of biochemical problems. Then, in 1944, he read a paper by Oswald Avery (P&S1904) of the Rockefeller Institute for Medical Research, now Rockefeller University. Dr. Averys research showing that DNA, not protein, transmitted genetic information stimulated Dr. 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.
MIT professor emeritus Boris Magasanik48 Ph.D., one of Dr. Chargaffs graduate students at the time, said the genetic material became the question that drove the lab on the 12th floor of Vanderbilt Clinic. Dr. Chargaff stopped his research on lipids and lipoproteins and switched full time to DNA research. At the time, Rockefeller biochemist Phoebus Levenes 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 organized in a way that was too simple to enable it to carry genetic information. Because of DNAs supposed simplicity, many people thought the chromosomes proteins, not its DNA, encoded the genes, even after Averys research. Averys finding was controversial and many people didnt want to believe it, Dr. Magasanik says. Chargaff believed Avery and wanted to show that DNA was more complicated than people previously believed. He wanted to show that DNA was different in different organisms; then it could very well be the genetic material.
Dr. 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 the Chargaff lab the opportunity to test the tetranucleotide hypothesis. First, Dr. Chargaff and his postdocs and students were able to adapt the technique of paper chromatography 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, the Chargaff 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 wasnt so simple after all.
Dr. 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 which came to be known as Chargaffs 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 Dr. 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, Dr. Judson reported in his book, The Eighth Day of Creation.
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 Dr. Watson didnt latch onto the idea that it could also mean the bases paired to each other until the very end of their model-building process.
In the meantime, though, Linus Pauling published his vision of the structure of DNA: a triple helix. Researchers Watson and Crick immediately saw errors in the model and continued on their own path. Pauling, when he was building his DNA structure, never thought about Chargaffs data, Dr. Watson said during a talk at Cold Spring Harbor Lab in 1999. Dr. Crick also later wrote that Rosalind Franklin and Maurice Wilkins at Kings College in London had a general idea what DNA looked like at the time but also overlooked the base rules. They had done no proper model building, Dr. Crick wrote in Nature in 1974. Mainly because of this they had missed the pairing of the bases and they had completely overlooked the significance of Chargaffs Rules.
Months after the meeting with Dr. Chargaff, Dr. Watson worked with cardboard cutouts of the bases, trying to fit them into a helical model of DNA revealed by X-rays taken by Rosalind Franklin. In a 2002 interview with filmmakers from Windfall Films and Cold Spring Harbor Laboratory, Dr. Watson recalled, Francis kept telling me theres Chargaffs pairs; would they pair to each other? But I didnt like Chargaff, ever since I had met him a year before. I thought: I dont want to use his data in finding the structure. Boy, it was really stupid. But I couldnt 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, Drs. Watson and Crick published their structure of DNA in Nature and cited Chargaffs work. Chargaffs 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, the Robert W. Johnson Jr. Professor Emeritus and Chairman of Biochemistry at P&S from 1977 to 1988. Dr. Edelman, who knew Dr. 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.
Although Dr. Chargaff didnt win a Nobel, his and other New York-based contributions to the discovery of the double helix were celebrated in February 2003 at the 50th anniversary DNA gala at the Waldorf-Astoria. The event was attended by Dr. Watson, several other Nobelists, and scientists from around the country.
Dr. Erwin Chargaff retired from Columbia in 1974 but maintained a lab at Roosevelt Hospital for several years. He died on June 20, 2002.
Susan Conova is a staff writer in the Office of External Relations at Columbia Health Sciences.