What are heterochromatin and euchromatin?

Chromatin is uncondensed nuclear DNA, the typical DNA morphology in interphase (the phase of the cell cycle in which the cells is not separating itself). In this phase of the cell cycle chromatin can be found as heterochromatin, more condensed and dark (in electronic microscopy) portions of DNA molecules, and as euchromatin, less condensed and lighter portions of DNA molecules.

As it is uncondensed the euchromatin is the biologically active portion of the DNA, i.e., the region that has active genes to be transcripted into RNA. The heterochromatin shows the inactive portions of the DNA molecule.

Cell Nucleus Review - Image Diversity: heterochromatin   euchromatin