Heterochromatin

Recall that, in eukaryotes, chromatin exists in two major forms based on its compaction level - euchromatin, and heterochromatin.

The heterochromatin is further divided into constitutive heterochromatin, and facultative heterochromatin.

Constitutive heterochromatin is a repeat-rich, gene-poor, and highly compacted region. Under a microscope, constitutive heterochromatin appears darkly stained, as the compaction allows it to take up more DNA binding dye.

A methylated histone tail characterizes constitutive heterochromatin. Methylation increases the affinity between histones and DNA, thereby increasing the chromatin compaction and inhibiting access to DNA.

The methylated histones are also bound by a nonhistone protein called Heterochromatin Protein 1, which facilitates chromatin compaction and spread of constitutive heterochromatin.

Facultative heterochromatin is a repeat-poor and gene-silent region. Under the microscope, it also appears darkly stained due to its higher compaction. The key distinction between facultative and constitutive heterochromatin is that the genes contained within facultative heterochromatin regions are flexible.

For example, in one cell, the genes in facultat