Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [6]
The genes of host and parasite are so similar, in fact, that scientists discovered certain kinds of viruses that could merge into E. coli, blurring their identities. These prophages, as they are called, can invade E. coli and then disappear. A prophage’s hosts behave normally, growing and dividing like their virus-free neighbors. Yet scientists found that the prophages survived within E. coli, which passed them down from one generation to the next. To rouse a prophage, the scientists needed only to expose a dish of infected E. coli to a flash of ultraviolet light. The bacteria abruptly burst open with hundreds of new prophages, which began to infect new hosts, leaving behind the clear pools of destruction. Two had become one, only to become two again.
THE STUFF OF GENES
In the merging dance of E. coli and its viruses, the Phage Church discovered clues to some of life’s great questions. And for them there was no greater question than what genes are made of.
Until the 1950s, most scientists suspected that proteins were the stuff of genes. They had no direct evidence but many powerful hints. Genes exist in all living things, even bacteria and viruses, and proteins appeared to be in all of them as well. Scientists studying flies had located genes in the chromosomes, and chromosomes contain proteins. Scientists also assumed that the molecules from which genes are made had to be complicated, since genes somehow gave rise to all the complexity of life. Proteins, scientists knew, often are staggeringly intricate. All that remained was to figure out how proteins actually function as genes.
The first major challenge to this vague consensus came in 1944, when a physician announced that genes are not in fact made of protein. Oswald Avery, who worked at the Rockefeller Institute in New York, studied the bacteria Pneumococcus. It comes in both a harmless form and a dangerous one that can cause pneumonia. Earlier experiments had hinted that genes control the behaviors of the different strains. If scientists killed the dangerous strain before injecting it into mice, it did not make the mice sick. But if the dead strain was mixed with living harmless Pneumococcus, an injection killed the mice. The harmless strain had been transformed into pathogens, and their descendants remained deadly. In other words, genetic material had moved from the dead strain to the live one.
Avery and his colleagues isolated compound after compound from the deadly strain and added each one to the harmless strain. Only one molecule, they found, could make the harmless strain deadly. It was not a protein. It was something called deoxyribonucleic acid, DNA for short.
Scientists had known of DNA for decades but didn’t know what to make of it. In 1869, a Swiss biochemist named Johann Miescher had discovered a phosphorus-rich goo in the pus on the bandages of wounded soldiers. The goo came to be known as nucleic acid, which scientists later discovered comes in two nearly identical forms: ribonucleic acid (RNA) and deoxyribonucleic acid. The phosphorus in DNA helps form a backbone, along with oxygen and sugar. Connected to this backbone are four kinds of compounds, known as bases, rich in carbon and nitrogen.
DNA was clearly important to life, because scientists could find it in just about every kind of cell they looked at. It could even be found in fly chromosomes, where genes were known to reside. But many researchers thought DNA simply offered some kind of physical support for chromosomes—it might wind around genes like cuffs. Few thought DNA had enough complexity to be the material of genes. DNA was, as Delbrück once put it, “so stupid a substance.”
Stupid or not, DNA is what genes are made of, Avery concluded. But his experiments