DE-NOVO CHROMOSOME FORMATIONS BY LARGE-SCALE AMPLIFICATION OF THE CENTROMERIC REGION OF MOUSE CHROMOSOMES

Chromosomes formed de novo which originated from the centromeric regio n of mouse chromosome 7, have been analysed. These new chromosomes wer e formed by apparently similar large-scale amplification processes, an d are organized into amplicons of similar to 30 Mb. Centromeric satell ite DNA was found to be the constant component of all amplicons. Satel lite DNA sequences either bordered the large euchromatic amplicons (E- type amplification), or made up the bulk of the constitutive heterochr omatic amplicons (H-type amplification). Detailed analysis of a hetero chromatic megachromosome formed de novo by an H-type amplification rev ealed that it is composed of a tandem array of 10-12 large (similar to 30 Mb) amplicons each marked with integrated 'foreign' DNA sequences at both ends. Each amplicon is a giant palindrome, consisting of two i nverted doublets of similar to 7.5-Mb blocks of satellite DNA. Our res ults indicate that the building units of the pericentric heterochromat in of mouse chromosomes are similar to 75-Mb blocks of satellite DNA f lanked by nonsatellite sequences. We suggest that the formation de nov o of various chromosome segments and chromosomes seen in different cel l lines may be the result of large-scale E- and H-type amplification i nitiated in the pericentric region of chromosomes.