Underrepresentation of nuclear DNA sequences in differentiating root cells ofVicia faba

Summary Data from cytological and biochemical analyses are presented on the behaviour of nuclear DNA during the differentiation ofVicia faba root cells. From the terminal 10.5 mm of the root, three segments were dissected by cutting transversely the root at 0.5 (segments I, meristematic cells), 4.5 (segment II, both meristematic and differentiating cells) and 10.5 mm (segment III, differentiating and/or differentiated cells) from the tip. Cytophotometric determinations of Feulgen absorptions in cell nuclei of the three root segments, carried out in preparations subjected to hydrolysis curve, revealed a lesser amount of nuclear DNA in differentiating cells when compared to the meristematic ones. Analyses of the reassociation kinetics of the DNAs extracted separately from the three root segments showed differences in the frequency of highly repeated sequences, which amount to 11.0, 8.6, and 7.5% of the total DNA in segments I, II, and III, respectively. Density gradient centrifugations in CsCl revealed a lighter satellite in the DNAs from segments I and II (ca. 5.4 and 3.8% of the total DNA, respectively) and no satellite in the DNA from segment III. It is suggested that underrepresentation of repeated DNA sequences occurs in differentiating cells and is a determining factor of the discharge of a cell from the mitotic activity.