H3-thymidine Autoradiographic Studies on the Heteropycnosis, Heterochromatin and Euchromatin in Spiranthes sinensis

Spiranthes sinensis, Orchidaceae, had large heteropycnotic condensed bodies in interphase nuclei and had large heterochromatic segments in the proximal region of the long arm of somatic chromosomes. Present paper deals with the pattern of DNA synthesis in the condensed bodies, heterochromatin and euchromatin through an autoradiographic study with H3-thymidine.1. Condensed bodies in interphase nuclei did not synthesize DNA. DNA in condensed bodies was found to replicate at the extended state of chromatin during early stage of DNA synthetic phase.2. Condensation of chromatin in interphase nuclei started at middle stage of DNA synthetic phase.3. DNA in euchromatin was found to replicate during the stages from middle to late DNA synthetic phase.4. DNA synthesis in the chromosomes initiated in the proximal heterochromatic region of long arm and ended at euchromatic short arm and at euchromatic distal region of long arm. In the heterochromatic region the occurrence of DNA synthesis. was found to be multicentric.5. Mitotic cycle phases were as follows in minimum duration: pre-DNA-synthetic phase, 4hrs.; DNA synthetic phase, 9hrs.; post-DNA-synthetic phase, 6hrs.; mitotic phase, 3hrs.; minimum mitotic cycle, 22hrs.6. The interphase nuclei in the root tip meristematic cells of this species were classified into 5 categories by morphological and physiological characteristics: micro- condensed nuclei, pre-DNA-synthetic phase; extended nuclei, early DNA synthetic phase; partly condensed nuclei, middle DNA synthetic phase; major condensed nuclei, . late DNA synthetic phase; macro-condensed nuclei, post-DNA-synthetic phase.