For decades, biology textbooks have enshrined a simple rule: DNA is made by copying a template. After one enzyme unzips a DNA double helix into separate strands, another called a polymerase builds a complementary sequence, base by base, for each strand. Presto: two copies of the original DNA. But new research into how bacteria defend themselves from viruses now shows this synthesis rule isn’t absolute. Today in Science , a Stanford University team describes a bacterial enzyme that synthesizes DNA without a nucleic acid template, using its own structure as a guide.

“The research is groundbreaking,” says Philip Kranzusch, a biochemist at Harvard Medical School who studies bacterial defenses. “Pretty cool!” adds Adi Millman, a computational biologist at the Massachusetts Institute of Technology. The use of a protein as a template for DNA synthesis, she says, “is a meaningful conceptual shift from the classical central dogma,” in which information flows in one direction from nucleic acids like DNA to protein.…