PLASTIDS AND MITOCHONDRIA: INHERITABLE SYSTEMS.

Evidence has been cited which indicates that RNA and DNA are present in plastids and mitochondria. A multigenic apparatus in the plastid is deduced from the properties of bleached Euglena strains. Control mechanisms are present for the differentiation of proplastids to chloroplasts in Euglena and in higher plants, and for the differentiation of promitochondria to mitochondria in yeast. An operon-regulator mechanism for this control is suggested. A comparison of the hereditary cytoplasmic units of Euglena plastids and yeast mitochondria indicates cates great similarities in their properties. Because of these similarities in two unrelated organisms, we suggest that a DNA unit which is self-duplicating and which serves as a code for RNA is the basic hereditary unit of each plastid and mitochondrion. Much work must be done if this reasonable hypothesis is to be converted into well-founded theory. Some pressing problems await solution. We do not understand the nonrandom distribution of plastids in the mitotic divisions of variegated plants. A related unresolved problem is that of maternal inheritance, in which nonrandom segregation of cytoplasmic organelles occurs after fertilization, causing elimination of the organelles which are contributed by the male parent. How different are the gene components of one plastid in a cell from the gene components of other plastids in the same cell, and how do we test for these differences? Can gene exchange or recombination occur between organelles within the same cell? The answers to these questions may have to await development of more sophisticated techniques, such as the ability to transplant these organelles between different cells or to culture cellular organelles in vitro.