Premature separation of neocentric metaphase chromosomes. (a) BBB cells treated with colcemid for ~24 hours show highly condensed chromosomes with separated arms but attached centromeres, as indicated by immunofluorescence with anti-centromere protein (CENP)-C (green). Fluorescent in situ hybridization (FISH) probe RP11-46I10 (red) from band 13q32 is found on the q arm of the two normal chromosomes 13 (full arrows) and on both ends of symmetric invdup13q neocentromere chromosome (see Fig 1). The neocentric chromosome has separated into its two replicated chromatids (split arrows), each of which contains a single kinetochore (green) which colocalizes with one of the RP11-46I10 FISH signals. (b) Quantification of separation of normal chromosome 13 (black and gray) and invdup13q neocentric (dark and light blue) chromosome at various times in colcemid. IMS13q at 16hours: 8/52 normal separated, 26/52 neocentric separated. BBB at 2 hours: 4/92 (4.5%) normal separated, 15/51 (30%) neocentric separated; at 12 hours: 17/103 (17%) normal separated, 26/57 (46%) neocentric separated; at 24 hours: 140/358 (39%) normal separated, 145/210 (70%) neocentric separated; at 36 hours: 41/108 (38%) normal separated, 39/56 (70%) neocentric separated.