We isolated and analyzed 19 NotI site-containing clones specific for human chromosome 21. The overall process consisted of selective isolation of Not I site-containing clones from flow-sorted chromosome 21 libraries, selection of independent clones by their restriction patterns and nucleotide sequences, and assignment of the clones to chromosome 21. Sequence analysis showed that the regions around the NotI sites had features typical of CpG islands, such as extremely high GC content, high-frequency appearance of CpG dinucleotide sequences, and lack of methylation of these CpGs. PCR conditions for these extremely G+C-rich templates were optimized to establish these NotI sites as sequence-tagged sites.
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