Studies on mitochondrial structure and function in Physarum polycephalum. IV. Mitochondrial division cycle.

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Abstract
The mitochondrial division cycle and morphological changes of the mitochondria in the plasmodium after fusion of microplasmodia of P. polycephalum were studied with acid fuchsin staining and light microscopic autoradiography. The number of mitochondria in the plasmodium increases during late S and early G2 and doubles at intervals of approximately 14 h. The mitochondria are classified into 3 types, small spherical, oval and dumbbell-shaped mitochondria. The frequencies of each type of mitochondria were examined at various times after fusion of microplasmodia. The results indicate that the decrease in the number of dumbbell-shaped mitochondria parallels the increase in the number of small spherical mitochondria and the relative number of mitochondria. It is also apparent that there is a sequential appearance of the dumbbell-shaped, the small spherical and the oval mitochondria. This suggests that the mitochondria multiply by division of dumbbell-shaped mitochondria. The light microscope, EM observations and light microscopic autoradiography showed that in fused plasmodia, a majority of the mitochondria synthesize DNA and divide during specific periods of the nuclear mitotic cycle. The existence of a mitochondrial mitotic cycle with periods, mitochondrial M (mM), mitochondrial G1 (mG1), mitochondrial S (mS) and mitochondrial G2 (mG2) is proposed. The durations (in hours) of the 4 phases are mM = 1.5, mG1 = 3.0, mS = 7.5 and mG2 = 2.0 and the generation time of mitochondria is 14 h.

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