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He focused on a particular mutation of the T4 virus called rII. This mutant had the useful quality of growing abundantly on one strain of the E. coli bacteria, strain B, while not growing at all on strain K. So a researcher could infect strain K bacteria with the mutants and watch for signs of T4. If any appeared, it must mean that something had happened to the rII mutation—presumably, it had reverted back to its original form. Such “backmutation” was relatively rare, but when it happened, giving the virus the ability to grow again in the K bacteria, it could be detected with extreme sensitivity, rates as low as one in a billion. Feynman compared finding a T4 backmutation to finding one man in China with elephant ears, purple spots, and no left leg. He collected them, isolated them, and injected them back into bacteria of strain B to see how they would grow.

Odd-looking plaques appeared. Among the normal, backmutated T4, he began to see phages that did not grow as they should have. He called them “idiot r’s.” He could only guess what might be happening at the level of the DNA itself to create the idiot r’s. He saw two possibilities: the site of the rII mutation in the DNA strand might have undergone a second, further mutation. Or a second mutation might have occurred at a different site, somehow acting to partially cancel the effect of the first mutation.

Tools for directly examining the genetic sequence, letter by letter, base pair by base pair, did not exist. But by painstakingly crossing the idiot r’s with the original virus, Feynman was able to show that his second guess was correct: two mutations, situated close to each other on the gene, were interacting. Furthermore, he showed that the second mutation had the same character as the first; it was another rII mutation. He had discovered a new phenomenon, mutations that suppressed each other within the same gene. Friends of his in the laboratory called these “Feyntrons” and tried to persuade him to write up his work for publication. Elsewhere, discovered independently, the phenomenon came to be called intragenic suppression. Feynman could not explain it. The Caltech biologists had no clear model for understanding how the genetic code was read, how the information encoded in DNA actually transformed itself into working proteins and more complex organisms. And Feynman’s time as a geneticist had come to an end. He desperately wanted to return to physics. When he was not grinding microsomes, he had been working more and more intently on a quantum theory of gravity.

Without realizing it, Feynman had come to the brink of the next great breakthrough in modern genetics. The specialists had an advantage after all: a year later, Francis Crick’s team at Cambridge, England, used the discovery of intragenic suppression as the touchstone for an explanation of how the genetic code was read. They guessed, correctly, that the mutations actually added or deleted a unit of DNA, thus shifting the message back or forward. One mutation threw the message temporarily out of phase; the next mutation put it back in phase. This interpretation suggested—or perhaps Crick already had it in mind—one of the simplest, yet strangest, mechanical models for genetic decoding: that the message of the gene is read in linear fashion, one base pair after another, from beginning to end. By 1966 Crick was declaring, “The story of the genetic code is now essentially complete.”

Ghosts and Worms

The problem of gravity had the finest pedigree—it came in a direct line of descent from Einstein’s greatest work—yet it lay outside the mainstream of high-energy theoretical physics in the early 1960s. As the general theory of relativity neared its fiftieth anniversary, some relativists and mathematical physicists continued to struggle with the natural problem of trying to create a quantum theory of gravitation—to quantize the gravitational field, as the fields associated with other forces had been quantized. It was difficult, involuted work.