title: Nucleocytoplasmic ratio requirements for the initiation of DNA replication and fission in Tetrahymena.
abstract: Hydroxyurea (10 mM) arrests the exponential growth of Tetrahymena by blocking DNA replication during S-phase. After removal of the hydroxyurea (HU), they have a long recovery period during which they are active in DNA synthesis. 3H-TdR uptake showed that on completion of the recovery period, the cells divide (recovery division) and enter a cell cycle which lacks G1. The frequency, size and DNA content of the extranuclear chromatin bodies (ECB) formed at this division are all markedly increased (2-4) over the corresponding values obtained from exponential growth phase controls. Microspectrophotometric analysis of macronuclear DNA content (N) coupled with the cytoplasmic dry mass (C) values suggest that specific N to C ratios (N/C) are required for the initiation of DNA replication and fission: during a normal (exponential growth) cell cycle, both N and C double, but asynchronously, so that the N/C of both post-fission-daughter cells and pre-fission cells is identical (standardized to N/C = 1) but late G1 cells have a low N/C. During a 10 hr exposure to HU, the N remains essentially the same whereas the C increases. When the HU is removed, the N increases by 4X and the C continues to increase until just prior to recovery division when it also reaches a value 4X that of the original daughter cells. Thus, the N/C = 1 is re-established. The enlarged ECB formed during recovery division may function to lower the N/C in the daughter cells, which in turn may in some way stimulate immediate DNA replication, thus eliminating G1. The elimination of G1 (and shortening in a few subsequent cell cycles) allows less time for cytoplasmic growth and results in the return of the cells to the generation time and the N and C values observed prior to the HU treatment.
mesh_terms: D000818:Animals; D002455:Cell Division; D002843:Chromatin; D003593:Cytoplasm / Q000032:analysis; D004247:DNA / Q000032:analysis; D004261:DNA Replication; D006918:Hydroxyurea / Q000494:pharmacology; D009940:Organoids; D013769:Tetrahymena pyriformis / Q000254:growth & development* / Q000378:metabolism; D013936:Thymidine / Q000378:metabolism
chemical_list: D002843:Chromatin; D004247:DNA; D013936:Thymidine; D006918:Hydroxyurea