Last time I described how each cell in your body has about 12 feet of DNA in its nucleus – a space about 6 microns in diameter. That your cells can efficiently find any gene or specific location amongst all that DNA is incredible, but the problem gets even worse when the cells goes to split in two: it has to make sure that each daughter cell gets one of every chromosome. To do this, cells coil the DNA into a tight bundle like an extension cord so that it can move it around and keep track of it more easily. But your cells are even better than that.
All along the length of that extension cord are genes, some of which are mission critical for keeping the cell alive, some of which aren’t (depends on the gene, depends on the cell). If a gene ends up in the middle of a bundled up extension cord, the gene is essentially turned off – the machinery that normally reads it can not get into the middle of that bundle, its just too tight. So what should a cell do with genes that it needs to express even during cell division (I got through graduate school thinking that gene expresion was turned off through mitosis, I haven’t seen any papers on this, but I am guessing that is no longer thought to be true)? It puts those on the outside of the bundle so it can still get at them.
To appreciate this, super glue about 100 really small solid gold coins to a 12 foot extension cord and coil it like you are at the cabin for the last time that summer and you want to show your dad that you can coil this thing so tight that the plastic fuses at a molecular level and will never ever come undone. (and ya ya, I know that the 12 feet is acutally in 46 separate segments, I am oversimplifying). Those 100 coins represent genes (I have no idea actually how many need to be expressed during division, I pulled the # out of my derrier), and you are out of luck my friend. You have coiled that thing so tight that you can’t possibly get at those coins. Your cells are dissapointed.
Your cells do better than that “around the forearm” coiling thing you do. Turns out they carefully loop the DNA so that each gold coin is on the outer surface of the chromosome. Now it can get at them whenever it needs to. Imagine having to do that every time you had to put away an extension cord. And imagine doing that trillions of times a day. Good job!