DNA is tightly packed in the form of condensed chromatin structure to form chromosomes. The nucleosome is the unit of chromatin that form from eight core histone (2 units of each H2A, H2B, H3, and H4) and 147 bp of DNA wrapped around it.
Some of the areas of chromatin are lightly packed. This lightly packed area is called euchromatin. On the other hand, the tightly packed area of the chromatin is called the heterochromatin. As the euchromatin is lightly packed, the DNA in that area is easily accessible by transcription machinery and transcription factors. Thus, euchromatin is the genetically active site of the chromosome.