II. REPLICATION OF DNA IN CHROMOSOMES 69 



chromosoiiies elongate far beyond the length ob.servcd at early meiotic 

 prophase (leptotene). The giant chromosomes of the Diptera provide 

 one example, but their structure is very complex and they contain many 

 chromonemata. In addition, since these giant chromosomes are end 

 products of differentiation, there may be some reason to question their 

 exact homology with mitotic chromosomes. However, in the oocyte of 

 amphibians a very much elongated chromosome exists which does revert 

 to a regular mitotic type at the following division (Fig. lA). These 

 chromosomes are 500-800 /,<. long in diplotene (Gall, 1956), but change 

 into chromosomes only 15-20 p. in length at later stages. In addition to 

 this shortening by a factor of 40-50 along the axis there are numerous 

 loops which are retracted into the main body of the chromosome. When 

 extended these would give the chromosome a contour length of several 

 centimeters. A few years ago Callan and McGregor (1958) showed that 

 DNase (deoxyribonuclease) would rapidly sever the loops of these 

 isolated lampbrush chromosomes. Although some cytologists had long 

 maintained that the loops were part of the chromatid axis, this was the 

 first clear demonstration that the genetic material, DNA, passed through 

 and maintained the continuity of the loops. In addition to the DNA of 

 the loops, the chromomeres along the axis, which consist of two closely 

 associated chromatids, were known to be Feulgen positive and therefore 

 to contain much of the DNA. This elongation to several hundred times 

 the length of a metaphase chromosome in itself leads one to suspect that 

 much of the bulk of even the early meiotic prophase chromosomes is 

 due to some type of folding or coiling of the much longer axis of one or 

 a few DNA strands. The difficulty of determining the structure of the 

 loops lies in the fact that a great amount of RNA and protein is attached 

 to them. In spite of this neither RNase (ribonuclease) nor proteases 

 will sever the loops. Recently Miller (1962) has been able to dissolve 

 off this protein and RNA coating with concentrated KCl solution and 

 to reveal the fine fibril, presumably DNA, which remains. Electron 

 micrographs reveal its diameter to be sometimes as small as 40-50 A 

 (Fig. IB). Gall (1962) has been able to obtain evidence, by the rate at 

 which the loops are broken by DNase, that the structural axis is com- 

 posed of a single DNA double helix. Since the loops are part of the axis 

 of single chromatids, these elongated structures which are among the 

 largest chromosomes in the animal kingdom are shown to have a single 

 DNA double helix as their linear component. DNase also produces 

 breaks between the chromomeres along the axis of the paired chromatids 

 with the kinetics predicted for a structure held together by a pair of 

 DNA double helices. If all of these results are correctly interpreted we 

 finally have the demonstration that the polymer maintaining the linear 



