title: Microspectrophotometric and autoradiographic study of the timing and duration of pre-meiotic DNA synthesis in Paramecium caudatum.
abstract: Timing and duration of pre-meiotic G1, S and G2 phases in Paramecium caudatum have been clarified by microspectrophotometry of DNA content and autoradiography with [3H]thymidine ([3H]dThd) and [3H]deoxyuridinemonophosphate ([3H]dUMP). Microspectrophotometric measurement of DNA content showed that cells in stationary phase have a G1 micronucleus, and that pre-meiotic DNA synthesis begins with the micronuclear swelling and ends immediately before the nucleus enters meiotic prophase. The results indicate that the micronucleus is in G1 during the first 1.5 h after onset of mating reaction at 25 degrees C and in pre-meiotic S phase during the following 1 h, and that G2 phase is very short or lacking. Therefore, in P. caudatum, the transition point from pre-meiotic interphase to meiotic prophase is at or very close to the end of pre-meiotic DNA synthesis. Autoradiographic study showed that although [3H]dThd and [3H]dUMP were well utilized in pre-mitotic DNA synthesis, only [3H]dUMP was well utilized in pre-meiotic DNA synthesis. [3H]dThd was utilized very little in pre-meiotic DNA synthesis even if the precursor was taken into the cell, suggesting that the activity of thymidine kinase is low in these cells.
mesh_terms: D000818:Animals; D001345:Autoradiography; D003227:Conjugation, Genetic; D004247:DNA / Q000096:biosynthesis*; D003856:Deoxyuracil Nucleotides / Q000378:metabolism; D007399:Interphase; D008540:Meiosis; D010247:Paramecium / Q000166:cytology / Q000378:metabolism*; D013053:Spectrophotometry; D013936:Thymidine / Q000378:metabolism; D013997:Time Factors
chemical_list: D003856:Deoxyuracil Nucleotides; D004247:DNA; D013936:Thymidine