A Possible Mechanism for the Replication of the Helical Structure of Desoxyribonucleic Acid.

1esoxyribonucleoprotein, the genetically active complex of the cell, carries on two important functions: that of self-replication and that of maintaining a cellular environment compatible with its own existence. An adjunct to the latter function among the differentiated cells of the multicellular organism is the accommodation of these processes within individual cells to the needs of the total organism. At present very little is known, at the molecular level, of the manner in which these processes take place. Recent advances in the knowledge of the chemical structure of this complex, however, make it possible to formulate a scheme for the replication of DNA which accords with the present-day conceptions of the complex and its cytological behavior during duplication of the chromosomal material prior to cell division. On the basis of X-ray diffraction studies and the chemical composition of the complex, the desoxyribonucleoprotamine molecule has been pictured by Feughelman et al.I as a triple coaxial, relationally coiled helix. Two relationally coiled polynucleotide strands (o and o' in Fig. 1, a and b) are connected across the center of the helix by hydrogen bonding through the bases, guanine being linked to cytosine and adenine to thymine as proposed by Watson and Crick.2 A third chain, a polypeptide, is relationally coiled about the two polynucleotide strands in such a manner that the E basic groups of adjacent amino acids, projecting in opposite directions but parallel to the axis of the helix, can form ionic linkages alternately with the phosphate groups of both polynucleotide helices. The two polynucleotide helices are therefore connected one with the other, both by hydrogen bonding through their complementary bases and by ionic linkage through the associated protamine (Fig. 1, a and b). The desoxyribonucleohistone complex is assumed here to be similar to the protamine counterpart. The differences between the protamines and histones are probably of degree rather than of kind. Histones are of an extremely variegated nature; their properties molecular weight,3 amino acid composition,4 even their occurrence in tissues5 appear to overlap somewhat with the less complex protamines. Both of these basic proteins form salt-like complexes with DNA. The protamine might be regarded as an extreme form of histone. The apparently globular nature of histone6 can be compared with the more extended protamine configuration by assuming the globularity of the former to be due to an abundance of nonbasic loops such as were postulated by Feughelman et al.' to occur to a limited extent even in the otherwise extended protamine. Replication of the polynucleotide in the desoxyribonucleoprotein complex can be visualized by first postulating a slight rotation or vertical displacement of one of the helihes relative to the other, the structure more closely approximating that originally proposed by Watson and Crick,2 in order to be able to accommodate additional strands of polynucleotide. Such a configuration would not contradict the results