The white locus is inactivated in a cell-by-cell variegated pattern when juxtaposed with the proximal or distal parts of the nucleolus organizer region (NO) by X chromosome inversion. Recombinants for two such inversions, w m5lb and w m4 , were obtained and randomized for genetic background. White locus activity was much higher in the w m4 recombinant duplicated for most of the NO and much lower in the w m5lb recombinant deficient for it. Although there may be other molecular differences between the heterochromatic regions of the recombinants, the most obvious is the dosage of NO. Suppression of a NO region-evoked variegated phenotype by additional NO doses is discussed in relation to four different classes of models for position-effect variegation (PEV): chromatin structure, nuclear geometry, incomplete transposition of mobile elements, and heterochromatin promoter-driven transcription. A corollary of the structural model is functional subdivision of heterochromatin, which would enable the use of PEV as a tool for its study.
The tritium is on the 4 (thymidine) or 4 and 5 (cytidine) carbon atoms of the pyrimidine nucleus. 20 Mitchell, H. K., personal communication. 21 The transformation of the soft, segmented larval cuticle into the hard, unsegmented cuticle of the prepupa, called the puparium, takes only a few minutes at 250C. The time at which the transformation occurred was known to the nearest half hour.
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