Continuous labelling of callus with H-thymidine results in intermittent peaks of
H-DNA per chloroplast, showing synchrony of division. The increase in H-DNA
could be due to several replication rounds, and the drop to successive plastid divisions
without intervening DNA synthesis. The level of DNA-binding proteins in the
chloroplast parallels the peaks of plastidal DNA synthesis; such proteins could hence
comprise factors necessary for plastid multiplication in tissue cultures