The remarkable fact that people who work on understanding our DNA always mention in their talks is that we have about 1.5 m of DNA in the nucleus of each of cells, and these nuclei are only around 5 thousandths of a millimetre across. That is a lot of DNA in a small space. Stretched out the DNA is longer than some of us are tall, but this DNA is crammed into a space too small to see with the naked eye.
This has been known for a long time, but until recently we knew little about the structure of this DNA in our cells. The basic picture we had was off a enormous amount of spaghetti crammed into a tiny space. But recently we have learnt that there is a tremendous amount of structure in the DNA, it is very highly organised – for reasons we only have a hazy understanding of.
One aspect of this structure that really fascinates me is how our genes move around. We know that most of our genes spend most of their time ‘off’ — in the sense that they are not making the RNA molecules that in turn make the proteins. Every so often, the cell needs more of a particular protein, and so the gene for that protein and the ‘factories’ for making these RNA molecules are brought together, across a distance of maybe half a micrometre (=thousandth of a millimetre). The RNA molecules that code for proteins are made in RNA ‘factories’ for these molecules, and these factories are typically working on around 10 or more genes at a time. The cell needs many different proteins so makes them on an industrial scale.
But this gene is part of a huge chromosome, along with hundreds of other genes, and the RNA factories are also pretty large. And remember that this chromosome and factory are squeezed into the nucleus with 45 other chromosomes and many other factories. So how is this movement done? I think we just don’t know.
It seems almost miraculous that these huge structures manoeuvre round in this tiny space. It must be like someone in the middle of a London underground carriage getting to the door of an absolutely packed carriage to get off the train. But they clearly do it, if they didn’t our cells would run out of protein and we would die.
Given how crowded the nuclei of our cells are, this movement must require forces being exerted on the factory and/or the genes, but where do these forces come from. There is a shortage of obvious candidates here. Outside the nucleus, we know a number of proteins can exert forces but these are exerted on filaments there, that appear to be absent inside the nucleus. This is a puzzle. But is a fascinating puzzle. I think scientists are going to have a lot of fun figuring out what is going on here.