The extent of chromatin compaction can be studied by staining chromatin using specific DNA binding dyes. Under the microscope, the dense-compacted regions take up more dye, appearing darker, while the less-compact areas take up less dye and appear lighter. Based on the compaction level, chromatins are classified into two primary forms – euchromatin and heterochromatin.
Euchromatin is the less dense region of the chromatin and stains lighter. Euchromatin contains histone H3 extensively acetylated on lysine at the 9th position of the histone tail region. Histones in the promoter region have methylated lysine 4 and phosphorylation of position 10. Extensive acetylation reduces the attraction between histones and DNA, loosening the chromatin. During interphase, the euchromatin can be found dispersed throughout the cell nucleus. It replicates during the entire duration of the S-phase of cell division.
