Studies on Mitochondrial Structure and Function in Physarum polycephalum. VI. Synthesis and Distribution of Mitochondrial DNA as Revealed by Light Microscope Autoradiography

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Abstract
The synthesis and distribution of mitochondrial DNA (mt-DNA) were studied by light microscope autoradiography in the plasmodium of P. polycephalum. Mitochondria pre-labeled with 3H-thymidine (3H-TdR) for 3 h during early mitochondrial S phase (mS) and incubated in non-radioactive medium showed elongated shapes that developed to dumbbell-shaped, and the latter form of mitochondria divided. The incorporation of 3H-TdR into mitochondria occurred during mitochondrial growth but terminated before development to dumbbell-shapes, suggesting that they synthesized mtDNA during a limited period of mitochondrial division cycle. Mitochondrial grains counts were reduced to about half during the late nuclear S phase, suggesting that Physarum mitochondria divided semi-synchronously during late nuclear S phase. The distribution of mtDNA pre-labeled during early or late mS was determined by the grain distribution of labeling of both poles of dumbbell-shaped mitochondria and labeling of daughter mitochondria. Apparently mtDNA was distributed equally to daughter mitochondria, regardless of time of mtDNA replication.