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>The brown Position-Effect Variegation as Related to Heterochromatin Region Association in Drosophila melanogaster
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The brown Position-Effect Variegation as Related to Heterochromatin Region Association in Drosophila melanogaster
The eukaryotic genome contains two types of cyto-logically distinguishable chromatin, euchromatin and heterochromatin. Unlike the transcriptionally active euchromatin, heterochromatin is epigenetically inactivated chromosome material that retains dense compaction irrespective of the cell cycle. Large blocks of het-erochromain consisting mostly of repeated DNA sequences are located, as a rule, in pericentromeric chromosome regions. In giant polytene chromosomes of Drosophila, in addition to the pericentromeric heterochromatin (PH), which forms a common chro-mocenter due to associations between the pericentromeric regions of all chromosomes, about 250 regions of intercalary heterochromatin (IH) are located in euchromatin [1]. IH forms dense discs thatstrongly resemble PH in several respects, such as late replication, DNA underreplication, and transcription inactivation: though, unlike PH regions, IH contains unique genes [2]. It is well known that, in part of cells, disturbance of the boundaries between euchromatin and heterochromatin near PH results in a spread of the "silent" state of heterochromatin to euchromatin genes. This phenomenon is referred to as positon-effect variegation (PEV). It remains unknown whether IH regions may also influence the expression of the genes transferred into IH or contacting it. This question was studied on a PEV model of the bwD gene rearrangement, which is induced by an insertion of a heterochromain 1.6-Mb satellite into the coding region of this gene [3]. In (bw~Dlbw~+) heterozygotes, this block of heterochromatin is assumed to be associated with PH and attract the normal bw gene allele, which is trans-inactivated because of the IH influence on the regulatory bw~+ regions [4-6]; this leads to eye-color mosaicism.
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