Definition of Heterochromatin
Heterochromatin is the form of chromatin which is very dense. It is tightly packed and is found in the nucleus of the eukaryotic cells. The DNA present in the heterochromatin is usually inactive transcriptionally. The condensed structure of the heterochromatin does not allow the access of DNA and RNA polymerases. Consequently, replication and transcription processes are prevented.
There are two kinds of heterochromatin that are constitutive heterochromatin and facultative heterochromatin. On the contrary, euchromatin is loosely packed and the DNA present in this region is allowed to be replicated as well as transcribed. The human chromatin is composed of 90% euchromatin and heterochromatin accounts for less than 10%.