28 Nature of the Genetic Material 



The recent work on lampbrush chromosomes (see I 2 A) makes 

 it probable that in the growing oocyte the chromonema may be re- 

 duced to a single-chain molecule, probably of nucleoprotein, and 

 there is cause to assume that the same is true of the preleptotene 

 chromosome. The reason is that it was found (Callan and others) 

 that the lampbrush chromonema, which (as part of a tetrad) should 

 consist of two chromatids, is single, showing that two chromonemata 

 may unite secondarily into one. If the one is actually a single-chain 

 molecule, remarkable conditions must prevail here at the molecular 

 level. Now it has been maintained by Kuwada (1940) — and there 

 are many facts in his favor — that a preleptotene chromosome can be 

 double (two chromatids and chromonemata) but afterward become 

 single again by union of the two chromonemata. Such facts will 

 have to be correlated (apart from their meaning for meiotic phe- 

 nomena) with the possible micellar structure of the chromosome at 

 other stages of its cycle. According to Guyenot and Danon (1953), 

 the electron microscope reveals that the single strand of the lamp- 

 brush chromosome contains two parallel threads; these are clearly 

 seen in the photos. Are these threads the molecular backbones of the 

 two united chromatids? Or could they possibly be related to the 

 two parallel chains of the Watson-Crick model of DNA? A diameter 

 of 100-150 A is given for one such "chromonema." The situation is 

 not made clearer by what Guyenot sees in the chromomeres and loops. 

 In the electron microscope the loops seem to consist of folding chains 

 of rods. It is assumed that they are surrounded by fibrous protein. 

 Only in the chromomeres — one per loop, called a "chromiole" — is 

 DNA attached on the outside. The fact (see Duryee, 1941; and Serra, 

 1947) that all DNA and RNA can be removed from these chromo- 

 somes by nucleases without disturbing their coherence shows that 

 the strands seen in the electron microscope cannot consist of nucleic 

 acid alone. Guyenot, if I understand him correctly, considers the 

 loops as products of the activity of the genes which take part in the 

 metabolic processes of the oocyte. Gall (1954) is of this opinion also. 

 Dodson ( 1948 ) showed that the loops start as Feulgen positive hairs 

 and later become DNA free loops. Since they can be dissolved with- 

 out damage to the chromosome (Gall), they are clearly different from 

 the chromonema and not loops of the chromonema as Bis assumed. 

 The lampbrush chromosomes still present many riddles (see Alfert's 

 review, 1954) and all our conclusions must be considered tentative. 

 But we may say at least that the facts thus far do not support the 

 idea of DNA as exclusive genie material. 



