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Cytoplasmic DNA in Differentiation and Development

Carnegie Institution of Washington, Baltimore, Maryland¹

Abstract

Our knowledge regarding cytoplasmic DNA may be summarized as follows. It is firmly established that the cytoplasm of frog and seaurchin eggs contain substantial amounts of DNA; in both species the bulk of this DNA is associated with mitochondria. There is ample evidence that the DNA is an integral component of these particles and that DNA is a general constituent of mitochondria of all tissues of all organisms so far studied. There may be smaller but still substantial quantities of non-mitochondrial DNA in egg cytoplasm, but little is known about this material at present.

The fate of mitochondrial DNA during development is not known, but it seems highly probable that it remains an integral part of the mitochondria and is distributed along with those particles into the cells of the embryo. Mitochondrial DNA is expected to be replicated whenever the multiplication of mitochondria is initiated during development.

In addition to DNA there is a substantial pool of low molecular weight deoxyribose-containing substances in eggs. Referring to this material as "cytoplasmic DNA" is highly illegitimate. The bulk of these substances may be monodeoxynucleotides, or derivatives thereof ("masked" nucleotides) in sea-urchin eggs. There is no compelling need to postulate the existence of oligodeoxynucleotides in eggs and a consideration of their function may be deferred until their presence is demonstrated.

The low molecular weight deoxyribose-containing substances of the egg are likely to constitute a precursor pool for the intense DNA synthesis during early development. Until some evidence to the contrary is presented, it may be assumed that DNA synthesis in the embryo proceeds in the usual way using deoxynucleosidetriphosphates.

¹ Department of Embryology.