April 19 2016 at 02:37AM

Tody I Learned: 1) In bacteria, the ratio of number of transcription factor to the number of its binding sites in the cell ranges from about 1/10 to about 10,000, trending around 10. That means that for a typical promoter, there are only about ten times as many copies of each transcription factor that can bind to it as there are places for them to bind. 2) Fountain pen nibs have a hole in the middle, kind of reminiscent of the sound holes in string instruments. These holes hold a film of ink, which is channeled through a slit down the nib to the point, which is how the pen deploys ink. If you press harder on the pen, the slit opens wider and lets more ink out, which is how you can vary the thickness of your stroke with a fountain pen. Thanks on this one to Mengsha Gong! 3) Ethidium bromide (EtBr, a DNA stain commonly used in laboratories and famed for its low cost, high sensitivity, and mutagenicity) is commonly injected into cattle to treat African Sleeping Sickness (trypanosomiasis). These cattle are injected at MUCH higher concentrations than are used in the lab (as in, 1000 times the concentration), and the cattle don't seem to show any ill effects. There is no reported carcenogenicity in ethidium-bromide-treated cows. Why are we lab scientists afraid of EtBr, then? Well, lab studies *have* shown that EtBr is a mutagen in cells in cell culture. And... that's about it. Chris Lennox gets the credit for this one!