This article provides notes about Heteropycnosis and its types!
It is a phenomenon shown by chromosomes in which certain regions of chromosomes stain more intensely than others.
The regions which exhibit heteropycnosis may be said to be composed of heterochromatin as described by Heitz (1928), and called heterochromatic parts against the other parts, which never show heteropycnosis and called euchromatic parts.
In 1928 Heitz defined heterochromatin as those regions of the chromosome that remain condensed during interphase and early prophase and form the so called chromocenters. The rest of the chromosome, which is in a non-condensed state, was called euchromatin (Gr., eu, true). Heterochromatin is closely associated with the nucleolus.
The heterochromatin controls the metabolism of chromosome, biosynthesis of the nucleic acids and the energy metabolism. Heterochromatins are usually located near the centromere (kinetochore), nucleolus organizer, and the ends of chromosomes, but are not restricted to these locations.
Two different types of heteropycnosis (Gr., hetero + pyknosis, different staining) are known.
1. Positive heteropycnosis:
In this, heterochromatic regions may be thicker and more dark-staining than the euchromatic ones. Here heterochromatin has synthesized more nucleic acid and possibly more protein as well. A positively heteropycnotic chromosome in a prophase nucleus looks like a metaphase chromosome.
2. Negative heteropycnosis:
In this, heterochromatic regions may appear thinner and stain more weakly than euchromatic regions in the same nucleus. Negatively heteropycnotic chromosomes usually have a fuzzy outline and at metaphase they still look like prophase chromosomes.
These both types of heteropycnosis may occur in the same chromosomes during different stages of cell division. For example, in short-horned grasshopper (Acrididae) X-chromosome passes through a regular cycle in the course of spermatogenesis, being negatively heteropycnotic during the early spermatogonial divisions (atleast during prophase and metaphase), non-heteropycnotic (i.e., behaving like euchromatin) in the later spermatogonial divisions and positively heteropycnotic during prophase of meiosis.
In heterochromatin the DNA remains tightly packed in 20 to 30 nm fibre which probably represents the configuration of transcriptionally inactive chromatin.