III. BACTERIOPHAGE DNA AND BACTERIAL DNA 117 



possible to conclude from these studies what proportion of the DNA 

 is oriented parallel to the phage tail. 



Similar studies have been made by Maestre and Kilksom (1962), 

 who took great care to avoid and to evaluate DNA ejection. Their 

 results for T2 and M-5 (a long-tailed phage) showed clear 3.36 A 

 reflections. The M-5 pictures also showed a strong 34 A meridional spac- 

 ing and a 52 A equatorial spacing. This confirms the notion that a 

 significant fraction of the DNA is oriented parallel to the phage tail. 



2. Partially Disrupted Phage Particles 



During the course of making electron micrographs of phage particles, 

 occasionally a disrupted particle can be seen with filamentous DNA 

 emerging from it. The first studies of this kind were done by Fraser 

 and Williams (1953). A tangled coil of filaments can be seen very close 

 to the protein ghost from which it presumably emerged. Rubenstein 

 (1960) noticed that phage particles could be broken by hydrodynamic 

 shear on the electron microscope grid in such a way as to draw out the 

 DNA from a tear in the head membrane. This treatment reproducibly 

 resulted in a long DNA fiber, presumably consisting of many DNA 

 molecules, with a characteristic blob or knot of material near the middle 

 of the strand. It is probably safe to conclude that this characteristic 

 knot is a result of some residual tertiary structure of the DNA. But 

 so far no direct information can be drawn about the tertiary structure 

 itself. It is possible that many types of internal organization could 

 result in such fibers with bulges. On the basis of careful measurements 

 of these strands and a consideration of packing requirements, Ruben- 

 stein made the suggestion that there exists a tertiary structure based 

 on the formation of cables from the single Watson-Crick duplex strands 

 (Fig. 2). 



3. The Electron Microscopy of Phage Maturation 



An important contribution to this problem was made by Kellenberger 

 et al., 1959; Kellenberger, 1962). By sectioning phage-infected bacteria 

 at intervals during intracellular growth, they find that by the 10th minute 

 each cell contains about 5 to 10 dense particles having the characteristic 

 shape of phage particles but somewhat smaller in size. While these dense 

 particles do not form in the presence of chloramphenicol, they are found 

 in abundance shortly after chloramphenicol is removed. On lysis of the 

 cell, these condensed particles are not found as intact phage, nor are 

 there enough "empty phage heads" to account for their number. One 

 cannot be completely sure that these condensed units are truly "mem- 

 braneless" for there is some evidence that the membranes may be broken 



