Distribution of interspersed repeats (Alu and Kpn) on NotI restriction fragments of human chromosome 21.

Interspersed repeated sequences (Alu and Kpn) were used as probes to detect a set of Not I restriction fragments of human chromosome 21 from the hybrid cell line WAV17. Forty different Not I fragments, ranging in size from less than 0.05 megabase (Mb) to 7.0 Mb, were identified. The total length of these fragments was 47.3 Mb. This length provides an estimate of the minimum size of the chromosome and a minimum number of fragments to be ordered to create a complete restriction map. The average length Not I fragment is 1.2 Mb. Alu and Kpn fragments are not always coincident: a 2.9-Mb fragment is detected with Kpn but not with Alu, and 13 fragments, ranging from less than 0.05 Mb to 5.6 Mb, are detected with Alu but not with Kpn; the 26 remaining fragments, covering 75% (35.3 Mb) of the total length, are detected with both repetitive probes. The presence of so many noncoincident fragments and the high variation of the hybridization signal intensities of the fragments suggest a very nonuniform distribution of Kpn and Alu repeats.