Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [3]
Proteins come in a maddening diversity of complicated shapes, but scientists discovered that they also share an underlying unity. Whether from humans or bacteria, proteins are all made from the same building blocks: twenty small molecules known as amino acids. And these proteins work in bacteria much as they do in humans. Scientists were surprised to find that the same series of enzymes often carry out the same chemical reactions in every species.
“From the elephant to butyric acid bacterium—it is all the same!” the Dutch biochemist Albert Jan Kluyver declared in 1926.
The biochemistry of life might be the same, but for scientists in the early 1900s, huge differences seemed to remain. The biggest of all was heredity. In the early 1900s, geneticists began to uncover the laws by which animals, plants, and fungi pass down their genes to their offspring. But bacteria such as E. coli didn’t seem to play by the same rules. They did not even seem to have genes at all.
Much of what geneticists knew about heredity came from a laboratory filled with flies and rotten bananas. Thomas Hunt Morgan, a biologist at Columbia University, bred the fly Drosophila melanogaster to see how the traits of parents are passed on to their offspring. Morgan called the factors that control the traits genes, although he had no idea what genes actually were. He did know that mothers and fathers both contributed copies of genes to their offspring and that sometimes a gene could fail to produce a trait in one generation only to make it in the next. He could breed a red-eyed fly with a white-eyed one and get a new generation of flies with only red eyes. But if he bred those hybrid flies with each other, the eyes of some of the grandchildren were white.
Morgan and his students searched for molecules in the cells of Drosophila that might have something to do with genes. They settled on the fly’s chromosomes, those strange structures inside the nucleus. When chromosomes are given a special stain, they look like crumpled striped socks. The stripes on Drosophila chromosomes, Morgan and his students discovered, are as distinctive as bar codes. Chromosomes mostly come in pairs, one inherited from each parent. And by comparing their stripes, Morgan and his students demonstrated that chromosomes can change from one generation to the next. As a fly’s sex cells develop, each pair of chromosomes embrace and swap segments. The segments a fly inherited determined which genes it carried.
There was something almost mathematically abstract about these findings. George Beadle, one of Morgan’s graduate students, decided to bring genes down to earth by figuring out exactly how they controlled a single trait, such as eye color. Working with the biochemist Edward Tatum, Beadle tried to trace cause and effect from a fly’s genes to the molecules that make up the pigment in its eyes. But that experiment soon proved miserably complex. Beadle and Tatum abandoned flies for a simpler species: the bread mold Neurospora crassa.
Bread mold may not have obvious traits such as eyes and wings, but it does produce many enzymes, some of which build amino acids. To see how the mold’s genes control those enzymes, Beadle and Tatum bombarded it with X-rays. They knew that when fly larvae are exposed to X-rays, the radiation mutates some of their genes. The mutations produce new traits—extra leg bristles or a different eye color—which mutant flies can pass down to their offspring.
Beadle and Tatum now created bread mold mutants. Some were unable to produce certain types of amino acids because they now lacked a key enzyme. But if Beadle and Tatum mated the mutant bread mold with a normal one, some of their offspring could make the amino acid once more. Beadle and Tatum concluded in 1941 that behind each enzyme in bread mold there is one gene.
A hazy but consistent picture of genes was emerging—at least a picture of the genes of animals, plants, and fungi. But there didn’t seem to be a place for bacteria in the picture. The best evidence for genes came from chromosomes, and bacteria