Measurable vs. Topological, PAMs, and The Speed of Sound

Today I Learned:
1) A measurable space is a space with a notion of measure, which means you can talk about the distance between two points in the space.

A topological space is a space with a notion of proximity, which means you can say that two points in the space are "close" (or not). Most measurable spaces are also topological, by virtue of having a measure.

2) ...the details of guide RNAs and tracrRNAs and PAM sequences in CRISPR/Cas. In particular, I cleared up some confusion over PAMs.... A PAM is a fixed sequence that has to be present in front of any sequence targeted by Cas9. The Cas9 enzyme, independently of any guide RNA, can unwind DNA, detect the PAM sequence, and start checking to see if it has a match to its associated guide. The PAM sequence varies by the species from which the Cas9 was derived, the most famous being the S. pyogenes Cas9 PAM, which is NGG (any nucleotide followed by two Gs). I was always confused about why there was an "N" in the specified sequence -- after all, if there can be any nucleotide there, why not just call the PAM "GG"? Well, turns out it's because the PAM is at the 3' end of the targeted sequence, not the 5' end, so the targeted sequence looks like

<sequence here>NGG

not

NGG<sequence here>

Also, I learned today that the PAM sequence does not appear in the guide, which makes sense.

3) The speed of sound in air is around 300 m/s (but varies by temperature, humidity, pressure, etc), which totally invalidates the advice I was once given about measuring distance to lighting by counting the seconds between a lightning bolt and its associated thunder. We'll see how long I remember this number.