April 14, 2016 at 02:26AM

Today I Learned: 1) Repressor proteins work best on weak promoters. In retrospect, the reason why is pretty obvious -- if the promoter is strong, it's because polymerases bind to it a lot, which makes it hard for a molecule to get in the way. This goes against my intuition, though, which is that if a promoter is weak, it doesn't have much room to go down in expression, so a strong promoter would be easier to repress. 2) One molar concentration (1 mole/liter) is approximately equivalent to 1 molecule in a box 1 nanometer on a side. My benchmark for stuff-that's-about-a-nanometer is the spacing between base pairs in a DNA helix, which is... 2 or 3 nanometers? Just a couple of nanometers. Anyway, that tells me that something at one molar would be concentrated enough to be peppered all over a strand of DNA just from randomly bumping into it -- that's pretty darned concentrated. Thanks Andy Halleran! Now I need to read the rest of "Cell Biology by the Numbers (http://ift.tt/1AvhcfE). 3) A colony of YFP*-positive bacteria glows quite nicely under a DNA gel light and filter. Also, cell-concentration GFP is pretty easy to visualize with a cell phone by taping the right filters over the flash and the camera. * YFP = Yellow Fluorescent Protein, which does exactly what it sounds like.