66 J. HERBERT TAYLOR 



constituent molecules and their disposition in space and time. Therefore, 

 we attack the problem by tr>-ing to understand the organization of the 

 cell's constituent molecular systems. 



In molecular genetics we might begin by trying to understand the 

 reproduction and organization of the genetic stmctures themselves — the 

 chromosomes. The tenii is used here in its broadest sense to designate 

 the entities in which is recorded the major part of the genetic infomiation 

 of cells and the more complex viruses. The smallest of these appear to 

 consist of a single molecule (Chapter IIIi. Whether we refer to a large 

 chromosome as a molecule or not may be a matter of semantics, but 

 evidence indicates that it has some of the properties usually associated 

 uith molecules — a definite size, atomic integrity over long periods of 

 time, and a regular sequence of its constituent parts which are bonded 

 together by rather stable chemical bonds. 



A basic hypothesis of molecular genetics is that the synthesis of DXA 

 is a process of replication, i.e.. that new molecules are made by copying 

 a complementary' sequence of the various nucleotides from an original 

 template which is not destroyed in the process. Much of the e\'idence 

 supporting this concept has been presented in Chapter I. The demon- 

 stration of the atomic integrity of two DXA subunits in a chromosome 

 (Taylor et al., 1957) and later in the various particles of DXA isolated 

 from chromosomes (Meselson and Stahl. 1958) was a significant con- 

 tribution to this concept, but these discoveries posed a larger question — a 

 problem of organization, packaging, and sorting that may not be solved 

 soon. The conserved subunits in the DNA particles are ver\- likely the 

 individual polynucleotide chains, although this interpretation has been 

 challenged (Cavalieri and Rosenberg. 1961a). Likewise the two con- 

 serv'ed subunits of a chromatid are almost certainly the same as those in 

 its fragments (the isolated DNA molecules or particles). Some of the 

 questions that are unresolved concern the continuity of the DXA along 

 the chromosome, the nature of linkers if these exist, the singleness or 

 degree of multiplicity of the chromonema. and the pattern of packaging, 

 folding, or coiling in the various structures that a chromosome assumes 

 during the phases of the cell cycle. In addition, the mechanisms which 

 regulate function and replication almost certainly have a basis in the 

 structure and organization of a chromosome, but the details arc largely 

 unknown. 



Three possible arrangements include most of the conceptual pictures 

 which have been presented: (/) a multistranded complex of DX'A. pro- 

 tein, and perhaps other constituents with many DX'A double helices 

 forming the axis of the chromonema: i2) some regular arrangement of 

 molecules of DX''.\ or of nucleoprotein particles linked together in tan- 



