Centromere (primary constriction):
The stained chromosomes reveal a certain region or constriction which stains lightly. This is called centromere or kinetochore or primary constriction. The centromeres occupy a constant position relative to the ends of chromosomes and are responsible for the various shapes of chromosomes. The centromere has a diameter of 0.2u and may occupy various positions on the chromosome. It (centromere) consists of spherules or granules (Kinosome) and interchromomeral fibrillae. The number of fibrillae per centromere varies. The chromonema are less coiled in the centromeric region and hence stain lightly.
Most of the organisms have their chromosomes with a single centromere (rnonocentnc). Certain other chromosomes have two centromeres (dicentric). In hemipteran insects however the chromosomes have many centromere (polycentric). Such as centromere is said to be a diffused centromere.
The proof for centromere being diffuse comes from the fact that when such a chromosome is fragmented by X-Rays, each bit functions like a typical chromosomes, replicates itself and participates in the division cycle. Diffused centromere has also been reported in Luzula purpurea of the family Juncaceae (Brown 1954). According to some cytologists, chromosomes with a diffused centromere are more primitive than the ones with a localized centromere.
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The polycentric chromosomes found in the nematodae Ascaris megalocephala are unique in the sense they are restricted to only those cells which constitute the germ line (Swanson 1957).