Figure 1.

Replication asynchrony at the 22q breakpoint. (a) A schematic diagram of the ends of both copies of the q arm of chromosome 22 in patient NT. The cosmid contig spanning the region appears above. The position of the distal end of cosmid N73A6 has been determined as indicated. The gray boxes represent telomeric (TTAGGG_n) repeats. The small arrowhead points to the breakpoint. CEN, centromeric direction. Scale is in kb. (b) FISH of NT nuclei using two probes. NT nuclei were hybridized simultaneously with digoxigenin-labeled cosmid N85A3 and biotin-labeled cosmid N85E7. Biotin was detected with FITC (green), and digoxigenin was detected with rhodamine (red). Cosmid N85A3 hybridizes only to the normal copy of chromosome 22 in NT cells, thus identifying the source of each signal as arising from the N85E7 probe. Before harvesting, cells were labeled with BrdUrd for detection of S phase nuclei. S phase cells were detected by an antibody to BrdUrd, which, in turn, was detected with AMCA (blue). The triple exposure in this figure represents an NT nucleus in S phase having replicated the N85E7 region in the normal chromosome 22 (green doublet) but not yet in the deleted chromosome (green singlet).