Who Needs This Junk, or Genomic Dark Matter.

Centromeres (CEN), pericentromeric regions (periCEN), and subtelomeric regions (subTel) comprise the areas of constitutive heterochromatin (HChr). Tandem repeats (TRs or satellite DNA) are the main components of HChr forming no less than 10% of the mouse and human genome. HChr is assembled within distinct structures in the interphase nuclei of many species - chromocenters. In this review, the main classes of HChr repeat sequences are considered in the order of their number increase in the sequencing reads of the mouse chromocenters (ChrmC). TRs comprise ~70% of ChrmC occupying the first place. Non-LTR (-long terminal repeat) retroposons (mainly LINE, long interspersed nuclear element) are the next (~11%), and endogenous retroviruses (ERV; LTR-containing) are in the third position (~9%). HChr is not enriched with ERV in comparison with the whole genome, but there are differences in distribution of certain elements: while MaLR-like elements (ERV3) are dominant in the whole genome, intracisternal A-particles and corresponding LTR (ERV2) are prevalent in HChr. Most of LINE in ChrmC is represented by the 2-kb fragment at the end of the 2nd open reading frame and its flanking regions. Almost all tandem repeats classified as CEN or periCEN are contained in ChrmC. Our previous classification revealed 60 new mouse TR families with 29 of them being absent in ChrmC, which indicates their location on chromosome arms. TR transcription is necessary for maintenance of heterochromatic status of the HChr genome part. A burst of TR transcription is especially important in embryogenesis and other cases of radical changes in the cell program, including carcinogenesis. The recently discovered mechanism of epigenetic regulation with noncoding sequences transcripts, long noncoding RNA, and its role in embryogenesis and pluripotency maintenance is discussed.