40 GENERAL CONCEPTS 



threads. Genetic and cytologic evidence indicates that the chromosomes 

 remain distinct physiologic and structural entities between successive 

 cell divisions even though they are not evident by the usual staining 

 procedures. 



It has been suggested that tlie chromomeres are, or contain, the 

 genes, for breeding experiments have shown clearly that these hereditary 

 units lie within the chromosome in a linear order. However, the correla- 

 tion between chromomeres and genes is not regular; some chromomeres 

 contain several genes and some genes have been located between chrom- 

 omeres. Several theories have been formulated to account for these 

 swellings of the chromosomes, but at present their true significance is 

 not clear. 



One of the very regidar characteristics of any kind of animal or 

 plant is the number of chromosomes in each nucleus. Every cell in the 

 body of every human being, for example, has forty-six chromosomes. 

 There are many other kinds of animals and plants which happen to have 

 46 chromosomes per cell as well; so the factor of chief importance in 

 differentiating different kinds of animals is not simply the number 

 of chromosomes per cell but the kind of genes in the chromosomes. The 

 chromosome number for most kinds of animals lies between ten and 

 fifty. One kind of roundworm has only two chromosomes per cell, certain 

 crabs have 200 and one kind of radiolarian, a marine protozoan, has 

 1600 or so chromosomes in its nucleus. 



Chromosomes occur in pairs; the forty-six chromosomes of each 

 human cell consist of two of each of twenty-three different kinds. The 

 chromosomes differ in length, shape, and in the presence of identifying 

 knobs or constrictions along their length. In most animals, the morpho- 

 logic features of the chromosomes are distinct enough so that one can 

 identify the individual pairs. 



Cell division must be an extremely exact process to ensure that each 

 daughter cell receives exactly the right number and kind of chromo- 

 somes. If we tamper experimentally with the mechanism of cell division, 

 and the resulting cells receive more or less than the proper number of 

 chromosomes, marked abnormalities of growth, and perhaps the death 

 of these cells, will follow. Mitosis may be defined as the regular process 

 of cell division by which each of the two daughter cells receives exactly 

 the same number and the same kind of chromosomes that the parent cell 

 contained. This process involves what appears to be a longitudinal split- 

 ting of each chromosome into two halves. There is now abundant 

 evidence that no such splitting can indeed occur; instead, each original 

 chromosome brings about the synthesis of an exact replica of itself 

 immediately beside itself. The new chromosome is made, some time 

 before the visible mitotic process begins, from raw materials present in 

 the nucleus. When the process is complete, the original and the new 

 chromosomes separate and become incorporated into different daughter 

 cells. The role of the complicated mitotic machinery is to separate the 

 "original" and "replica" chromosomes and deliver them to opposite ends 

 of the dividing cell so they will become incorporated into different 

 daughter cells. 



