MOLECULAR AND MACROMOLECULAR STRUCTURE 187 



The flagellum protein of certain flagellated bacteria is of the a-type and 

 each single flagellum is a whip-like thread whose dimensions are of the 

 same order as those of an a-filament in hair of which it may be regarded 

 as a structural analogue. Burge (1960) has inferred from the X-ray pattern 

 of isolated flagella (see Fig. 86) that the hexagonally close-packed bundles 

 of a-helices must consist of only a small number (3-7) of helices. There 

 must then be several of such bundles in parallel to form a structure as 

 large as the whole flagellum ( ~ 100 A) suggesting again the existence of 

 sub-filaments within a main a-filament. 



The Organization of <x-Filaments into Larger Structures 



On passing from the molecular level to the macromolecular and histo- 

 logical levels we enter the domain (see Fig. 1) where microscopy is able to 

 provide definite answers to structural problems. While these matters will 

 be more fully considered in the next chapter it will be useful here to discuss 

 some geometrical aspects of the larger structures. It is perhaps advisable 

 to emphasize the succession of structures of increasing size with which 

 we are concerned. In hair, for example, a-helices ( ~ 10 A diameter), 

 a-filaments (60-80 A diameter) and fibrils (0-05-1/n diameter). These are 

 depicted diagrammatically in cross-section in Fig. 80. 



Various and somewhat speculative schemes can be advanced to explain 

 the packing of the a-filaments as it is actually observed in the fibrils of hair 

 (Fig. 102, p. 247), wool and skin (Fig. 98, p. 225). The geometry of the 

 packing is very variable : in skin cells, an extreme case of irregular packing, 

 the filaments cluster in irregular-sized sheaves with no constant orientation 

 relative to each other but on the whole tending to lie in the plane of the 

 flattened cell (Fig. 99, p. 229). In hair, wool and quill, we find definite 

 macrofibrillar bundles of filaments oriented parallel to the fibre axis (Fig. 

 102, p. 247). In cross-section these show some variety of appearances 

 ranging from good hexagonal packing to spirals (see Plates 15 and 16) 

 the latter appearing more common in the middle stages of development of 

 the hair. Stacks of flat sheets may occur in the a-protein of the mantis 

 ootheca (see p. 205). 



If an a-filament (diameter < 100 A), the constructional unit of these 

 formations, is not circularly symmetrical but is polarized as suggested in 

 Fig. 81, the energy conditions governing the recruiting of new members to 

 an already-formed aggregate might well favour the development of flat 

 sheets, Fig. 81 (b) or (a) helical sheets. The stacking of one sheet against 

 another could be governed by conditions of the sort discussed by Crick 

 in connexion with the association of the smaller units, the a-helices, to 

 form filaments. That is, we may assume that the filaments themselves also 

 have helically-fluted surfaces (or lines of special attraction) in this case 

 arising from their construction as multi-stranded cables of a-helices, and 



