D NA encodes the amino acid sequences of each protein in our bodies, but carries no instructions for how much of each protein to make. Instead, a cell’s protein factories, its ribosomes, are set spinning by RNA, DNA’s single-stranded relative. By controlling when and which genes are transcribed from DNA to RNA, a cell regulates its protein production.
Transcription is carried out by an enzyme called RNA polymerase. RNAP wraps around DNA like a collar. As it proceeds along DNA, RNAP pulls the helix apart to expose a short stretch of the bases whose sequence embodies the genetic code. Then, from the surrounding solution, RNAP grabs free-floating bases and adds them in the proper, complementary sequence to the end of a growing chain of RNA.
Now, Steven Block of Stanford University in California and his collaborators have used an innovative optical trap to track the progress of a single RNAP...