II. REPLICATION OF DNA IN CHROMOSOMES 



71 



Fig. IB. Electron micrograph of a .small portion of a loop of a lampbrush 

 chromosome. Most of the protein and RNA has been dissolved away with a satu- 

 rated solution of KCl after placing an isolated chromosome on Formvar film. The 

 dissolution was interrupted by adding saturated (NH4):S04 at pH 5.1 followed by 

 staining with uranyl acetate (x 75,000). Thinner portion of axial fiber is 40-50 A in 

 diameter. (Original photograph contributed by O. L. Miller.) 



integrity of a chromatid is, at some stage anyway, DNA, and tliat a 

 single pair of polynucleotide chains from this axis. Linkers along the 

 axis are not ruled out but neither are any revealed by these experiments. 

 However, long stretches of peptides or RNA which might serve as link- 

 ing material appear unlikely from the fact that proteases and RNase 

 do not break the loops. 



In addition to the much extended chromosomes of the amphibian 

 oocytes, other oocytes and spermatocytes of many species have chro- 

 mosomes of a less extreme lampbrush type. These chromosomes as well 



