Expression of the mitochondrial genome in HeLa cells: I. Properties of the discrete RNA components from the mitochondrial fraction

Deoxyribonucleic acid biosynthesis in mitochondria. Purification and general properties of rat liver mitochondrial deoxyribonucleic acid polymerase.

When an organism has only one or two mitochondria or chloroplasts per cell, it is probable that their partitioning is always stringently controlled so that each daughter cell always receives half the organelles in the parent cell. When there are more copies of an organelle, the available data suggest that partitioning is stochastic but far from random, with a strong tendency toward equality. The molecular mechanisms that promote equal partitioning are not known in any case, but the great variety of organelle behavior suggests that many different mechanisms are involved in different organisms. As Wilson (1925) pointed out, the precision of partitioning of cytoplasmic organelles rarely if ever equals that of mitosis, but it is still an expression of selection for mechanisms that will ensure the hereditary continuity of the organelles. How cells compensate for unequal partitioning by controlling organelle replication is known for only one case. But when one considers that Tetrahymena and Paramecium use different methods to compensate for unequal partitioning of macronuclear DNA, it would not be surprising if organisms use a variety of different compensating replication modes for organelles as well. What is surprising is that so little attention has been paid to these problems. Nothing could be simpler than counting organelles in dividing cells, but this has been done on a large scale in only two systems. Quantitative techniques in cell biology have been developed to the point where such studies could be done even on cells that have too many organelles for direct counting. Molecular mechanisms of partitioning have scarcely been touched on. Much more has been done on the role of the cytoskeleton in determining cell shape, and some observations have been made on its role in positioning organelles in interphase cells, but these kinds of studies have not been extended to dividing cells. Some experiments and observations have been made on the role of microtubules and microfilaments in moving cytoplasmic organelles around the cell during interphase, but again nothing has been done on their possible role in partitioning organelles at cytokinesis. The major lesson of this article is how little has been done, and how much can be done. The partitioning of cytoplasmic organelles at cell division is a wide-open field for future research, and one of great importance for both genetics and cell biology.

The partitioning of cytoplasmic organelles at cell division.

Die Orthologie und Pathologie des Nukleinsäure- und Eiweißstoffwechsels der Zelle im Autoradiogramm

Mitochondrial DNA synthesis and the mitotic cycle in Physarum polycephalum

Autoradiographs of Tetrahymena cells labeled with 3H-thymidine for long periods of time indicate incorporation into acid-insoluble, DNase sensitive material in the cytoplasm. Results of electron microscope autoradiography show that 73% of the cytoplasmic silver grains occurred over mitochondria profiles, 92% of the grains were located over or within 0.5 μ of mitochondria while 97% of the silver grains were over or within 1.0 μ of mitochondria.



Tests to determine loss or gain of radioactivity during cell growth and division show that approximately one-half of the label in each cell is transmitted to each daughter cell during division.



The results demonstrate that mitochondria of Tetrahymena pyriformis contain a stable DNA-like material.

A stable mitochondrial DNA in Tetrahymena pyriformis

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A stable mitochondrial DNA in Tetrahymena pyriformis