The Chromosome and Its Division 7 



cytoplasm exists. Whether cytoplasmic heredity is independent of 

 the chromosomes and, if so, how it compares with chromosomal 

 heredity are special questions. (12) The genie material within the 

 chromosomes is self-duplicating and capable of mutating into a new 

 and again self-duplicating condition, and if genetic material exists 

 in the cytoplasm it must have the same characteristics. 



It is not difficult to derive a theory of the genetic material which 

 accounts for most of the basic facts in a formal way. In the classic 

 theory of the gene, a chromosome is a string of discrete bodies, the 

 genes, arranged in linear order within the framework of the chromo- 

 some. These genes are endowed with the power of self-dupHcation 

 as well as of specific attraction to their likes and repulsion to un- 

 likes, and they interact in some way with the cytoplasm as deter- 

 miners of reactions. It is the first problem of theoretical genetics to 

 scrutinize the details of chromosomal behavior which underlie these 

 basic facts, and to see whether the theory of the gene in its classic 

 formulation is a satisfactory description and a logical explanation of 

 the facts, or whether it will have to be replaced by a more compre- 

 hensive idea which accounts for more of the special facts. Thus we 

 must scrutinize the details of the cytological and genetical facts 

 pertinent to the formulation of a general theory to see whether they 

 point to a general principle which might or might not be the classic 

 theory of the gene. In doing so we shall have to touch upon many 

 facts without trying to catalogue them, but with the intention of not 

 missing any really relevant information. 



The first question is whether the known structure, chemistry, and 

 behavior of the chromosome shed any light upon its genetic organiza- 

 tion. Clearly, the basic fact of genetics is the ability of the chro- 

 mosome to reduplicate. If there were no genetic material to be 

 duplicated, the old description of the chromosome sphtting into 

 longitudinal halves would suflBce. The more recent additional details, 

 namely, the dividing of the centromere and the appearance of the 

 two coiled chromonemata, would not change the simple picture of a 

 more or less amorphic material growing to double size and just being 

 divided up. But this simple picture is no longer accurate when we 

 think of the genetic material in the chromosome. Whatever its chemi- 

 cal composition, an exact replica is required by the facts of genetic 

 constancy. 



Biochemists seem to be generally of the opinion that a large 

 molecule of the type probably constituting the genie material is not 

 synthesized independently in innumerable synthetic steps by the 



