Types of chromatin in different regions of the chromosome

(1) Telomeres. In some organisms evidence has been presented that the telomeres or ends of chromosomes contain heterochromatin or repetitive DNA. The evidence has been give mostly for plants. The telomeres probably serve as chromosome ends; maintain the individual integrity of each chromosome.

(2) Primary constriction (centromere).  The primary constriction or the cenetromere contains chromatin to which spindle fibers are attached. It is continuous with heterochromatin both sides.

(3) Secondary constriction I (the nucleolar organizer). The nucleolar organizer consists of euchromatin and is imagined to lie within the nucleolus. It contains the genes for the synthesis of 18S and 28S ribosomal RNA. RNA synthesis takes place throughout the cell cycle, except during mid-mitosis.

(4) Secondary constriction II. The secondary constriction II consists of euchromatin which carried out mRNA synthesis almost throughout the cell cycle, like the nucleolar organizer. It has been suggested that in at least some cases the secondary constrictions II may represent sites of 5S ribosomal RNA synthesis.

(5) Satellite. A mass of heterochromatin at one end of the chromosome represents the so-called satellite.

(6) Euchromatin. The bulk of the chromosome is made up of euchromatin. It is believed to be partly composed of non-repeated DNA sequences which synthesize mRNA during interphase.

(7) Intercalary heterochromatin (represented by dots and short segments in the figure) is found the euchromatin.