| Definition: |
The compaction of chromatin into a conformation that is refractory to transcription but that can be converted to euchromatin and allow transcription in specific contexts. These can be temporal (e.g., developmental states or specific cell-cycle stages), spatial (e.g., nuclear localization changes from the center to the periphery or vice versa due to exogenous factors/signals), or parental/heritable (e.g., monoallelic gene expression). In metazoa, this involves the methylation of histone H3K27, and in certain cases methylation of H3K9. On the other hand, in unicellular eukaryotes, only methylation of H3K9 is involved and H3K27 is not methylated. |