I’ve always been interested in the stories behind BioNumbers. Textbooks say DNA polymerase, for example, replicates 220 nucleotides per second. But how do we know this? How can one actually measure the speed of a single enzyme? A fast typist, for comparison, types ~80 words per minute or ~5-7 characters per second. This means that DNA polymerase (in this case, from T7 phage) “types” letters 30-40 times faster than a skilled typist. Measuring the speed of a single enzyme is not so simple. In this case, one cannot simply time how long it takes for a cell to divide, say, and then divide that time by the cell's genome length to arrive at an estimate of DNA polymerase speeds. That is because division rates are not bottlenecked by DNA division times, and bacterial cells initiate multiple rounds of DNA replication on their genomes simultaneously. Therefore, we need another way. Ideally, there would be a “single molecule” way to observe DNA replication happening without relying upon highly indirect measurements. In 2006, a group of Harvard biochemists figured out a way to do exactly that. Before explaining their experiment, though, it’s important to note that these biochemists took advantage of length differences between single-stranded and double-stranded DNA. Double-stranded DNA has a rigid, helical structure (which we learn about in school) and, therefore, also a consistent length of about 0.34 nanometers per base pair. Single-stranded DNA, for comparison, is far more flexible and disordered. It doesn’t have stabilizing hydrogen bonds like double-stranded DNA does, so it tends to coil and collapse on itself. Under experimental conditions, and outside of cells, single-stranded DNA has a short length of only about 0.03 nanometers per nucleotide. Now, here’s how this experiment worked: First, the researchers took a long, single-stranded thread of DNA and stretched it between a glass surface and a tiny bead, floating in liquid. Next, they added a short piece of DNA, called a primer, which latched onto the DNA near the glass surface. This gave the enzyme a starting point to begin copying DNA. Then, they added the T7 DNA polymerase to the liquid. As the enzyme added new nucleotides, it turned the single-stranded DNA into stiff, double-stranded DNA. The DNA began to uncoil and stretch out. And since one end was fixed to the glass and the other to the bead, the bead slowly moved as the DNA extended. Finally, the researchers watched this movement using a microscope and camera. By tracking the bead’s movements, and measuring its distance further and further from the glass slide, they figured out that polymerase was copying about 220 nucleotides of DNA per second. From this experiment, the researchers could ALSO see that the DNA polymerase enzyme moves in "bursts;" it doesn't copy DNA continuously. This is just one “Behind the BioNumbers” experiments that I really enjoy. I’ll plan to write more about these in the future if you enjoy them.