COMPLEMENTARY REPLICATION R-BAND AND G-BAND PATTERNS INDUCED BY CELL BLOCKING AT THE R-BAND G-BAND TRANSITION, A POSSIBLE REGULATORY CHECKPOINT WITHIN THE S-PHASE OF THE CELL-CYCLE/

The characteristic band patterns of replication banding (dynamic bandi ng) were analyzed. High-resolution (550-1,250 bands per haploid genome ) G- and R-band patterns were obtained after 5-bromo-2'-deoxyuridine ( BrdU) incorporation during early or late S phase. Thymidine-BrdU permu tation culture methods, which arrest DNA synthesis at the R-band/G-ban d transition, allow complementary BrdU substitution. The RBI (R bands by BrdU using immunological staining) and GBI (G bands by BrdU using i mmunological staining) band patterns were complementary for all chromo somes. There was no overlapping, and every part of each chromosome was positively stained by one of the two banding procedures. Comparative analysis of RBG (R bands by BrdU using Giemsa staining) and RBI band p atterns, as well as GBG (G bands by BrdU using Giemsa staining) and GB I band patterns, showed good congruency, displaying a very good band-f or-band match. The congruency and complementarity found for these band patterns show that high concentrations of both thymidine and BrdU blo cked S-phase progression near the R-band to G-band replication transit ion within the S phase. They also prove that BrdU incorporation is com plementary and, therefore, demonstrate the existence of the R/G transi tion; a possible regulatory checkpoint within the S phase of the cell cycle.