Fig. 3. DNA synthesis during development. DNA synthesis during development was monitored by BrdU incorporation. (A) High molecular DNA was separated on standard pulsed-field gels and BrdU was detected by immunostaining after Southern transfer of the DNA. The amount of BrdU incorporation into chromosomes was compared to level of labeling observed in growing cells during one cell cycle and normalized to the amount of chromosomal DNA on the blot, as detected with a chromosomal probe (B). See text for details.