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

Abstract

ABSTRACT. The synthesis and distribution of mitochondrial DNA (mt-DNA) were studied by light microscope autoradiography in the plasmodium of Physarum polycephalum. Mitochondria pre-labeled with 3H-thymidine TdR) for 3 h during early mitochondrial S phase (mS) and incubated in nonradioactive medium showed elongated shapes that developed to dumbbellshaped, 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. In addition, mitochondrial grains counts were reduced remarkably to about half during the late nuclear S phase, suggesting that Physarum mitochondria divided semisynchronously during late nuclear S phase. The distribution of mtDNA prelabeled during early or late mS was determined by the grain distribution of (a) labeling of both poles of dumbbell-shaped mitochondria, and (b) labeling of daughter mitochondria. The results indicated that mtDNA was distributed equally to daughter mitochondria, regardless of time of mtDNA replication.