From Scientific American:
For decades the cell nucleus has been a black box of biology—scientists have understood little about its structure or the way it operates. But thanks in part to new visualization technologies, biologists have recently begun probing the architecture of the nucleus in real time. And they are discovering that this architecture appears to change as we age or fall ill or as our needs shift. In fact, the structure of nuclear components—chromosomes, RNA, protein complexes and other small bodies—could be as biologically important as the components themselves.
…Chromosomes position themselves carefully relative to one another, too. Mouse olfactory cells contain the genes for 1,300 types of smell receptors, but only one of the genes turns on in each cell. In a 2006 paper researchers used fluorescent tags to show that a receptor gene becomes expressed only if it comes into physical contact with a specific part of chromosome 14. The idea is that “these two chromosomes come together in three-dimensional space, they kiss, and that’s how you get your regulation” of genetic activity, Misteli says. Chromosome “kissing” also appears to play a role in determining which X chromosome gets turned off in female cells, because only one copy is usually active.