Gene activation and deactivation related changes in the three-dimensional structure of chromatin.

Chromatin in the interphase nucleus is dynamic, decondensing where genes are activated and condensing where they are silenced. Local chromatin remodelling to a more open structure during gene activation is followed by changes in nucleosome distribution through the action of the transcriptional machinery. This leads to chromatin expansion and looping out of whole genomic regions. Such chromatin loops can extend beyond the chromosome territory. As several studies point to the location of transcription sites inside chromosome territories as well as at their periphery, extraterritorial loops cannot simply be a mechanism for making transcribed genes accessible to the transcriptional machinery and must occur for other reasons. The level of decondensation within an activated region varies greatly and probably depends on the density of activated genes and the number of engaged RNA polymerases. Genes that are silenced during development form a more closed chromatin structure. Specific histone modifications are correlated with gene activation and silencing, and silenced genes may become associated with heterochromatin protein 1 homologues or with polycomb group complexes. Several levels of chromatin packaging are found in the nucleus relating to the different functions of and performed by active genes; euchromatic and heterochromatic regions and the models explaining higher-order chromatin structure are still disputed.