CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 183 



theless, it is possible that a single-stranded nucleoprotein backbone, 

 similar to that of the lampbrush chromosome, but coiled and contorted 

 in specific regions, mav represent the common basis of chromosome or- 

 ganization. Recent attempts to correlate chromosome structure with 

 genetic and autoradiographic findings have taken into account only the 

 number of strands required by the data. This turns out to be one DNA 

 double helix per interphase chromosome before duplication. On this 

 scheme, what cytogeneticists call the half-chromatid is equivalent to a 

 single DNA strand. We shall reconsider chromosome structure at the 

 end of this chapter. 



INTEGRATION OF DNA IN THE BACTERIAL CELL 



Studies of the macromolecular architecture of the bacterial cells are 

 mostly very recent, and as yet no definitive picture has emerged. Here 

 we shall present a current view which may change as more data become 

 available. 



Bacterial DNA is seen under the light microscope as Feulgen-positive 

 material. The region where it is found corresponds to an area, as seen 

 in sectioned cells in the electron microscope (Plate II), of very low 

 electron density, containing or consisting of fine fibers 30-60 A in 

 diameter, probably highly hydrated. There is an abrupt transition be- 

 tween this area and the surrounding dense cytoplasm, but no evidence 

 has been obtained of a nuclear membrane. Cells have been observed 

 at all stages of division, but no apparent differences have been noted 

 in the physical state of the DNA which might correspond to the con- 

 densation and coiling of chromosomes in higher organisms. 



In fact, nothing comparable to mitosis has been seen in bacteria, in 

 the opinion of most competent cytologists. 



Since it has been established that the DNA forms a complex with 

 histones in most cells of nuclear organisms and with protamines in some 

 sperm, it was of particular interest to determine whether bacterial DNA 

 also existed as a nucleoprotein complex. In a recent study, DNA was 

 extracted from E. coli under extremely mild conditions in an attempt 

 not to dissociate any existing DNA-protein complex. The extracted 

 DNA was found in association with a nonhistone protein. X-ray dif- 

 fraction studies of the complex indicated that at least half of the DNA 

 was not bound to the protein, and that a large part of the protein was 

 not bound to the DNA. 



The striking common feature of all these systems is their fibrillar 

 organization: whether DNA or nucleoprotein, the material consists of 



