Figure 2.

A model showing a way in which specific interchromosomal colocalization could be facilitated by associations with a common nuclear body. Each panel shows a mouse cell nucleus; the territories of chromosomes 10 and 11 are indicated, with loops representing gene loci on each chromosome. Small circles represent transcription factories and NB indicates another nuclear body. (a,b) In a one-step model there is no correlation between the positions of the two chromosome territories in the nucleus (a), but genes loop out from the chromosome territories and colocalize as a result of interacting with a shared nuclear body (NB), with or without an associated transcription factory (b). But it is difficult to imagine how loci on two different chromosomes can find each other with the high efficiency that is observed if their chromosome territories are randomly distributed. (c,d) A more plausible two-step model in which the two chromosome territories are brought into the same general vicinity of each other, perhaps through association with the same nuclear body (c). Specific colocalization of genes on the two chromosomes is then established (d), perhaps by association with the same nearby transcription factory.