The Gene: An Intimate History - Siddhartha Mukherjee Page 0,95

between bacteria. Kornberg had added enzymes to it and made it replicate in a test tube. To insert a gene into the SV40 genome, all that Berg needed was a series of reactions. He needed an enzyme to cut open the genome circle, and an enzyme to “paste” a piece of foreign DNA into the SV40 genome necklace. Perhaps the virus—or, rather, the information contained in the virus—would then spring to life again.

But where might a scientist find enzymes that would cut and paste DNA? The answer, as so often in the history of genetics, came from the bacterial world. Since the 1960s, microbiologists had been purifying enzymes from bacteria that could be used to manipulate DNA in test tubes. A bacterial cell—any cell, for that matter—needs its own “tool kit” to maneuver its own DNA: each time a cell divides, repairs damaged genes, or flips its genes across chromosomes, it needs enzymes to copy genes or to fill in gaps created by damage.

The “pasting” of two fragments of DNA was part of this tool kit of reactions. Berg knew that even the most primitive organisms possess the capacity to stitch genes together. Strands of DNA, recall, can be split by damaging agents, such as X-rays. DNA damage occurs routinely in cells, and to repair the split strands, cells make specific enzymes to paste the broken pieces together. One of these enzymes, called “ligase” (from the Latin word ligare—“to tie together”), chemically stitches the two pieces of the broken backbone of DNA together, thus restoring the integrity of the double helix. Occasionally, the DNA-copying enzyme, “polymerase,” might also be recruited to fill in the gap and repair a broken gene.

The cutting enzymes came from a more unusual source. Virtually all cells have ligases and polymerases to repair broken DNA, but there is little reason for most cells to have a DNA-cutting enzyme on the loose. But bacteria and viruses—organisms that live on the roughest edges of life, where resources are drastically limited, growth is fierce, and competition for survival is intense—possess such knifelike enzymes to defend themselves against each other. They use DNA-cutting enzymes, like switchblades, to slice open the DNA of invaders, thereby rendering their hosts immune to attack. These proteins are called “restriction” enzymes because they restrict infections by certain viruses. Like molecular scissors, these enzymes recognize unique sequences in DNA and cut the double helix at very specific sites. The specificity is key: in the molecular world of DNA, a targeted gash at the jugular can be lethal. One microbe can paralyze an invading microbe by cutting its chain of information.

These enzymatic tools, borrowed from the microbial world, would form the basis of Berg’s experiment. The crucial components to engineer genes, Berg knew, were frozen away in about five separate refrigerators in five laboratories. He just needed to walk to the labs, gather the enzymes, and string the reactions in a chain. Cut with one enzyme, paste with another—and any two fragments of DNA could be stitched together, allowing scientists to manipulate genes with extraordinary dexterity and skill.

Berg understood the implications of the technology that was being created. Genes could be combined to create new combinations, or combinations of combinations; they could be altered, mutated, and shuttled between organisms. A frog gene could be inserted into a viral genome and thus introduced into a human cell. A human gene could be shuttled into bacterial cells. If the technology was pushed to its extreme limits, genes would become infinitely malleable: you could create new mutations or erase them; you could even envision modifying heredity—washing its marks, cleaning it, changing it at will. To produce such genetic chimeras, Berg recalled, “none of the individual procedures, manipulations, and reagents used to construct this recombinant DNA was novel; the novelty lay in the specific way they were used in combination.” The truly radical advance was the cutting and pasting of ideas—the reassortment and annealing of insights and techniques that already existed in the realm of genetics for nearly a decade.

In the winter of 1970, Berg and David Jackson, a postdoctoral researcher in Berg’s lab, began their first attempts to cut and join two pieces of DNA. The experiments were tedious—“a biochemist’s nightmare,” as Berg described them. The DNA had to be purified, mixed with the enzymes, then repurified on ice-cold columns, and the process repeated, until each of the individual reactions could be perfected. The problem was that the cutting enzymes had not been optimized, and the yield was minuscule. Although