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b, Sequence organization in human subtelomeric DNA regions. The terminal repeat tract consists of 2–15 kb of simple repeat sequence (TTAGGG)n and is indicated by the black arrow at right. Short (50–250 bp) and often degenerate (TTAGGG) tracts (internal black arrows) are highly enriched (.25-fold) in subtelomeric DNA relative to elsewhere in the genome. A subtelomeric repeat (Srpt) region (blue) consists of a mosaic patchwork of segmentally duplicated DNA tracts that occur in two or more subtelomere regions and range in size from ,10 kb to .300 kb. TAR1, D4Z4 and beta satellite sequences are frequently associated with Srpt regions. Proximal to the Srpt region is chromosome-specific genomic DNA, typically with a high GC content and high gene density. Stretches of segmentally duplicated DNA that occur only once within subtelomeric regions (tan) are interspersed with 1-copy subtelomeric DNA (yellow) in a telomere-specific fashion. Overall, segmentally duplicated DNA comprises approximately 25% of the most telomeric 500 kb of the chromosome, a fivefold enrichment over the genome-wide average. |