. Biological structure and function; proceedings. Biochemistry; Cytology. 528 D. M, PRESCOTT fixed, extracted to remove acid-soluble material, and autoradiographed. Within only i -5 min. after addition to the medium, pH]-cytidine is taken up, converted to the appropriate form and incorporated into nuclear RNA (Fig. 2(a)). After 5 min. the rate of pH]-cytidine incorporation into the nuclear RNA rises. Incorporated activity is not detected in the cytoplasm until about 12 min.; in contrast, the nucleus is densely labelled (Fig. 2(6)). After 12 min., label accumulates steadily in cytoplasmic RNA b
. Biological structure and function; proceedings. Biochemistry; Cytology. 528 D. M, PRESCOTT fixed, extracted to remove acid-soluble material, and autoradiographed. Within only i -5 min. after addition to the medium, pH]-cytidine is taken up, converted to the appropriate form and incorporated into nuclear RNA (Fig. 2(a)). After 5 min. the rate of pH]-cytidine incorporation into the nuclear RNA rises. Incorporated activity is not detected in the cytoplasm until about 12 min.; in contrast, the nucleus is densely labelled (Fig. 2(6)). After 12 min., label accumulates steadily in cytoplasmic RNA but the rate of accuitmJation of radioactivity in the nucleus recedes to a lower value at about 25 or 30 min. At 35 min. the nucleus and cytoplasm are equally labelled (Fig. 2(r)), and at 60 min. the cytoplasm contains more than twice as much label as the nucleus. The nucleus at this time, however, is still (60- |120-. 40- 25 30 35 TlME(min) Fig. I. The two curves show the time course of the total amount of [^H]-cytidine incorporated into RNA of the nucleus and cytoplasm of Tetrahymena with the isotope continuously present in the medium. Each point is the mean grain count for autoradiographs for 23 to 26 cells. The range for each point indicates 95% confidence limits. more densely labelled. Ribonuclease digestion shows that DNA synthesis contributes very little to this incorporation. L nlabelled deoxycytidine has been added to the medium with the intention of minimizing pH]-cytidine entrance into DNA in all experiments. The time of appearance of tritium in cytoplasmic RNA varies from one experiment to another. In one case it occurred slightly earlier than 13 min. and in another experiment did not begin until 25 min. In the latter experiment, the longer delay is probably related to a short interruption in cell proliferation imposed by transfer of the log phase cells to fresh nutrient medium just before the experiment was Please note that these images are extracted from sc
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