Finally, the day you have looked forward to for months – the annual meeting of the American Ball of String Society. You wake up early that morning, as you still have to pack. You aren’t even bringing a change of clothing – just your string. Approximately half a mile of string to carefully coil into your suitcase.
This is sort of similar to something every one of your cells does every time it divides. If you stretched out the DNA from one cell in your body, it would stretch about 12 feet (note, you obviously have this broken up into 46 chromosomes, so you would have 46 pieces totalling 12 feet). Maybe thats not impressive, but I bet this is: your average cell nucleus is 6 microns in diameter – 6,000 times smaller than that suitcase. Imagine keeping that organized (but you don’t have to imagine it, you are doing it right now all over your body. Good job!).
How does it do this? Well, to greatly simplify, the cell does what you do with a long extension cord: it coils the DNA into a compact structure that takes up less space. This compact structure is the structure that you would probably call a chromosome, but in fact that piece of DNA is always a chromosome – its now called a mitotic chromosome. It coils that thing like my Dad did – tightly; so tightly that genes stuck in the middle are essentially prevented from being expressed.
The problem of keeping this all organized is even harder than that. All along the chromosomes are your genes. Depending on which tissue the cell is a part of, these genes can either be really really important (like a gene that codes for a protein that repairs mutated DNA) or not so very important (like the insulin gene if you are a neural cell in your brain). Genes are read and expressed by special protein enzymes, and these special protein enzymes need space to work. But when you coil up the chromosome like an extension cord, what happens if the important genes, the ones that *that* cell needs to constantly express ends up in the tightly packed interior? That question will have to wait until next time.
This post was inspired by a recent paper I read in PLOS Biology. Next up: what fruit flies can tell us about packing chromosomes into tiny little suitcases.