1 



THE CHROMOSOME 

 AND ITS DIVISION 



An inquiry into the nature of the genetic material must start with the 

 following basic facts. (1) The chromosomes are the structural ele- 

 ments which from bacteria to man are in control of the major features 

 of heredity. (2) All chromosomes are similar in structure and be- 

 havior. (3) On the morphological as well as the genetical level, 

 chromosomes are largely constant within a given species. This in- 

 cludes their "individuality," meaning their actual continuity through 

 all phases of cell life. (4) Chromosomes are able to duplicate and 

 the two duplicates are normally identical, morphologically and ge- 

 netically. (5) Chromosomes in diploid organisms, or diploid stages 

 of organisms, consist of two sets of identical partners. (6) These 

 homologues are endowed with the ability to synapse, undergo meiosis, 

 and exchange segments at definite stages of the cellular cycle. (7) 

 Chromosomes have in addition to their visible structure a genetical 

 structure, which is strictly polarized: this linear diJBFerentiation is 

 completely orderly from point to point in a definite and constant 

 pattern, which duplicates exactly in division. On the morphological 

 level this is often visible as a constant arrangement of different 

 segments or chromomeres and also as the typical point by point 

 synapsis in meiosis. On the genetical level it is expressed in the well- 

 known arrangement of genetic loci or genes on the chromosome map. 

 (8) Chemically, chromosomes are always a combination of proteins, 

 only a few of which are known, and desoxyribonucleic acid (DNA), 

 both together being the chromatin of the cytologist. A varying amount 

 of ribonucleic acid (RNA) is also present. (9) There is a chemical 

 interaction between chromosomes and cytoplasm involving the nucleic 

 acids, especially RNA. (10) The chromosomes are capable of ab- 

 normal behavior: breakage, abnormal distribution, and so on. The 

 genetic consequences are always those expected on the basis of the 

 known genetic, polarized structure. All the facts of classic genetics 

 may thus be described as a result of the distribution of the chromo- 

 somes and their parts, which is an expression of the statistical con- 

 sequences of chromosomal behavior in meiosis and fertilization. (11) 

 The cytoplasm must play a role in heredity, since no nucleus without 

 6 



