July 10, 2016 at 04:08AM

Today I Learned: 1) ...how to keep a credit score high. Most important thing is to pay your bills on time, but it's also good to have multiple credit cards, keep multiple accounts open of different types, keep accounts open for a long time, and keep your credit draws well below your credit limit. None of which are things I do (except for paying bills on time), yet my credit score is pretty fine, so I'm not sure what's up with my credit score. 2) No, X-Gal will not dissolve in 5% DMSO in water. As much as I want it to, it just won't. 3) Speaking of X-Gal, it turns out that most plasmids used for blue-white screening... wait a second, let me back up a bit. Yesterday I explained what X-Gal is. In brief, it's a molecule that turns blue when the enzyme β-galactosidase is present. Why would you want such a molecule, though? Well, the answer is that it's useful for diagnosing certain failure modes when constructing a plasmid (a circular piece of DNA with up to a few genes). There are lots of ways to build a plasmid, but most of them go something like this: you start with a pre-existing plasmid that produces some sort of antibiotic resistance (called a "vector") and some kind of DNA you want to insert on the plasmid; you mix the two with some enzymes that will hopefully insert your DNA into the vector; you forcibly insert whatever you get into a bunch of bacteria and grow them up on agar plates at low density, along with an antibiotic that the plasmid confers resistance against; each bacteria that got a whole plasmid will survive the antibiotic purge and grow up into a visible colony of a whole bunch of bacteria; you can then pick that colony (as in, literally pick up with a pipette tip), grow it up in media, and harvest your new plasmid. One of the (many) ways plasmid construction can go wrong is that the original vector will sometimes just survive the construction process and get into a bacteria unmodified. Blue-white screening is a way of quickly identifying which bacteria have unmodified plasmids. To do blue-white screening, you start with a vector that expresses β－galactosidase, and you do your construction in a way that, if done correctly, will remove or destroy the β－galactosidase gene. Then, when you grow up your plasmid-transformed bacteria, you do it on plates with X-gal, so that the colonies with unmodified plasmid look blue. So, that's blue-white screening. Today I learned that most vectors for blue-white screening have β-galactosidase under a lac promoter, which is naturally repressed. To actually produce the β-galactosidase and turn colonies blue, you have to *also* add IPTG, a molecule that lifts lac repression. I tried making some stocks of mixed IPTG and X-Gal solution, then used that to make plates for blue-white screening. We'll see how well they work.