MOBILE GENETIC ELEMENTS, CHIASMATA, AND THE UNIQUE ORGANIZATION OF BETA-HETEROCHROMATIN

Beta-heterochromatin in Drosophila and the Syrian hamster share a simi lar DNA organization, few unique sequences, and scrambled repeats of m obile elements without tandem repetition. DNA in alpha-heterochromatin is tandemly repetitious, and we now show that the repeat unit can eit her contain or lack a mobile element. The tandem repeat organization o f alpha-heterochromatin is presumably due to a concertina-like mechani sm of unequal exchange between repeat units. Although both heterochrom atin types are late replicating and can incorporate mobile retroposons , the sequence distinction between the two heterochromatins appears to be due to a property conferred by chiasmata upon the process of homol ogous recombination in beta-heterochromatin but not in alpha-heterochr omatin. Chiasmata seem to suppress the concertina mechanism of unequal exchange and impart to beta-heterochromatin its non-tandem, scrambled repeat organization.