Euchromatin and heterochromatin:
Based on the staining reaction two regions may be identified in the chromatin material. These are Euchromatin and Heterochromatin.
The euchromatin stains positively with the DNA specific stains (basic fuchsin) indicating a concentration of DNA. This region is genetically active and stains lightly. The euchromatic regions are supposed to represent areas of less condensation.
Heterochromatin stains more deeply than euchromatin and represents highly condensed regions on the chromosome. In the interphase nucleus, the heterochromatic regions form condensed structures called chromocentres or false nucleoli. The following table summarises the differences between euchromatin and heterochromatin
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Types of Heterochromatin:
There are two main types viz., constitutive heterochromatin and adaptive heterochromatin. The first type shows dark staining reaction (Heteropycnosis) also and in all cells, while the second type shows heteropycnosis only in certain cells or only at certain stages in the cell cycle. Constitutive heterochromatin was originally named satellite DNA (S-DNA).
It is mostly inactive during protein synthesis and is repetetive from the point of view of base sequence. Adaptive heterochromatin is metabolically inert and comprises of about 2.5% of genome.
Heteropycnosis:
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This is the staining phenomenon seen in the chromosomes with reference to heterochromatic and euchromatic regions. The heterochromatic regions which stain deeply are said to exhibit heteropycnosis as against euchromatin which stains lightly.
However it should be noted that the heterochromatin always does not stain deeply with reference to euchromatin. While the euchromatic region is always constant in its behaviour (non heteropycnotic) towards staining (has constant coils), the hetrochromatic region may stain deeply or lightly and this is known as differential heteropycnosis. Differential heteropycnosis is of two types-positive and negative.
The areas of heterochromatin showing positive heteropycnosis are more condensed and deeper staining than euchromatic region. In areas of positive heteropycnosis there are more of nucleic acids and proteins. Chromosomes with positive heteropycnosis in prophase appear like metaphase chromosomes.
The areas of heterochromatin showing negative heteropycnosis are possibly less condensed and stain lighter than euchromatic region. Chromosomes with negative heteropycnosis have a hazy outline and metaphase chromosomes appear like prophase chromosomes.