[Heterochromatin and centromere structure paradox]

Centromere (CEN) is the structure responsible for the chromatid association, chromosome attachment to the spindle, and correct position in the plate. The only DNA found in the mammalian CEN belongs to the satellite DNA--high repeated tandem repeats. Mounting evidence indicates that both types of chromatin (CEN and peri-CEN) are required for proper centromere function. CEN, peri-CEN and peritelomeric regions remain white spots at the chromosome maps appeared after reading genomes of human, mouse, and rat. SatDNA is considered to be species-specific. Library hypothesis regards heterochromatin as the library of different satDNA one fragments of which became spread and fixed in species fixation. We have analyzed database Chromosome Unknown (ChrUn) and found several new classes of mouse tandem repeats. The features of these classes are similar with the ones from rat ChrUn, as well as their distributions according to GC-richness. We believe that similar fragments' structure, i. e. intermingling of fragments with different curvature rather than their primary sequence will help to solve the paradox, when CEN or peri-CEN fragments from different animals have nothing in common, but bind the same sets of proteins.