Cells of a choline-requiring mutant of Neurospora crassa, labeled with radioactive choline, were transferred to unlabeled medium. At various times during their subsequent logarithmic growth, a highly purified mitochondrial fraction was prepared by sucrose density gradient centrifugation, and the distribution of label among individual mitochondria was determined by quantitative autoradiography. Preliminary experiments indicated that, under the conditions of this "washout" experiment, choline served as a stable mitochondrial label. Radioautographic analysis showed that, in fully labeled mycelia and for three mass doubling cycles in the unlabeled medium, radioactivity was randomly distributed among all mitochondria; i.e., the distribution of autographic grains among individual mitochondria followed a Poisson distribution. In experiments in which pulse labeling for 10 minutes was used, the label was randomly distributed among all mitochondria. The data suggest that the mitochondrial mass is increased by a continuous process of addition of new lecithin units to the already existing mitochondrial framework.

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