Graduate Thesis Or Dissertation
 

The effects of various factors on the formation and fate of extranuclear chromatic bodies in Tetrahymena pyriformis

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  • During karyokinesis in the ciliate, Tetrahymena pyriformis, strain GL-I, an extranuclear chromatin body (ECB) is frequently formed as an excluded mass between the two dividing daughter macronuclei. The formation and fate of ECB's were followed by means of Feu lgen, toluidine blue 0, and euchrysine staining and autoradiography (³H-thymidine). The following results were obtained: 1) Fifty-one percent of the dividing cells in a log growth phase culture produce one (43%) or more (8%) ECB's. 2) Newly formed ECB's respond to stains and extractions as do macronuclei. 3) Single ECB's assort randomly into either daughter cell. 4) By the completion of cytokinesis, most newly formed ECB's have undergone changes in morphology and stainability. 5) The ECB's gradually disappear during the cell cycle: the percentage of daughter cells with recently formed ECB's remains relatively constant at 30% for the first 35 minutes and then decreases gradually to about 10% just before division 2 (at 205 minutes). 6) The remaining ECB's are carried through division 2, and are found in 5% of the subsequent G₁ cells. 7) Most of these ECB's disappear during the next cell cycle, although some are carried into division 3. 8) ECB's once formed do not incorporate labeled thymidine at any time in the cell cycle. ECB's in log growth phase G₁ cells placed into starvation medium (inorganic salts) disappear rapidly and almost completely (from 30% at division to 1% by 90 minutes as compared to 30% to 15% in cells maintained in nutrient medium). Almost all (88%) heat synchronized cells form at least one ECB at the first synchronized division (21% form two), resulting in 48% of the daughter cells with an ECB. This value drops rapidly to 8% within 80 minutes and to 5% just prior to the second synchronized division (at 105 minutes) and then rises to 55% following this division. The extra percent of ECB's is due to the small carryover from the first division. The loss of ECB's probably results from intracellular digestion with subsequent reincorporation or elimination of the breakdown products. These and other data suggest that ECB formation is a factor in the maintenance of some optimum nucleo-cytoplasmic mass ratio which may be involved in the control of the cell cycle.
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