Over Christmas I try to do a bit of reading, of papers I may not have time to think about during term time. I am reading a couple (1,2) of papers by McKnight and coworkers in the biology journal Cell. In a nutshell, they have discovered, at least partially by accident it appears, that important biological molecules called messenger RNAs may be transported along the long spines of nerve cells in small jelly containers. By small I mean less than a thousandth of a millimetre across, which is small but still big enough to contain a lot of molecules.

Basically, McKnight and coworkers show that there are protein molecules in our nerve cells that can make a jelly, a bit like the jelly we might have in a trifle. Jellies are a type of substance known as a gel. Most of a gel is actually a liquid, water in the case of these containers and in the case of trifle jelly, but despite this gels are mechanically a solid, i.e., do not flow like a liquid but have a definite shape that can resist the pull of gravity. So they are a solid, but they are typically soft, a trifle jelly is a lot softer than steel.

So, even as you read this, tiny little jelly blobs are acting as mini packing crates inside the nerve cells in your brain. The messenger RNA molecules are packed into these jelly crates and then transported to where they are needed. The question then is: Why make these packing crates out of jelly?

Presumably, evolution has selected jelly because it is good in some way. It may be that jelly is strong enough to hold the cargo molecules, but soft enough to be easily dissolved when the cargo reaches its destination – jelly is easy to dissolve, for example, trifle jelly becomes liquid when it is heated. But that is just speculation, and it is purely qualitative. It would be good to have a quantitative model of what our nerve cells are up to here, but that is a bit tricky. The experiments of McKnight and coworkers are mostly in the contents of cells that basically have been put in a mixer and then studied in a test tube. Studying these little jelly blobs as they zip around inside nerve cells is hard.