Markierungsmuster nach H3-Thymidineinbau in Kernen der Hypokotylzellen von Sinapis alba L.

DNA-synthesis in the hypocotyls of Sinapis alba L. was studied with H³-thymidine labelling. Cells from hypocotyl segments were stained by the Feulgen-method and squash preparations were made. The following labelling patterns were observed: 1. Labelling of the chromocentres only. 2. Nuclear area evenly labelled. 3. No radioactivity in the chromocentres. This pattern was rarely seen. — The frequency of the first two types in different tissue segments is not equal. In segments with more differentiated cells there was an increase in the percentage of nuclei with radioactivity only in the chromocentres. This could be due to a prolongation of the phase of synthesis in the chromocentres in this tissue. — The total number of labelled nuclei decreases basipetally as well as with the age of the hypocotyl. In hypocotyls of seedlings older than 52 hrs radioactivity appeared only sporadically in the nuclei. The decrease in the number of labelled nuclei is faster than the decline of the corresponding measurable total DNA synthesis in the hypocotyl. This can either be due to extra nuclear DNA synthesis or depend on an increase in DNA synthesis in the later replicating heterochromatic region of the nucleus.