232 



with the microscope structures in the 

 cell whose behavior exactly corres- 

 ponded to the behavior of the genes. 

 It had been found that certain bodies 

 which had been termed chromosomes 

 were indeed divided equally whenever 

 a cell divided, each chromosome be- 

 coming split longitudinally, one half 

 going to each daughter. These and no 

 other bodies in the cell were distributed 

 with exactitude to the daughter cells. 

 It was found that the number of chro- 

 mosomes in a cell corresponded ex- 

 actlv to the number of blocks of genes 

 existent in that cell, that larger blocks 

 of genes corresponded to larger chro- 

 mosomes and vice versa. The behavior 

 of the chromosomes at the time of re- 

 productive cell formation also fitted 

 exactly that of the genes, so that the 

 chromosomes were distributed to the 

 germ cells according to the same rules 

 which governed the distribution of the 

 genes. Furthermore, whenever a ease 

 was found where the chromosomes be- 

 haved in an unorthodox manner in this 

 regard, it was found that the genes be- 

 haved in corresponding fashion, and 

 whenever genes were found to have 

 changed places, via inversion or trans- 

 fer of segments, cytologists were able 

 to demonstrate that corresponding seg- 

 ments of certain chromosomes had 

 suffered the same alteration. 



All of this has demonstrated be- 

 \ond question that the genes are car- 

 ried in the chromosomes and that the 

 laws of the distribution of genes from 

 parent to offspring are in reality the 

 laws governing the distribution of the 

 chromosomes from one generation to 

 the next. 



The biochemical approach has 

 shown that the chromosome is com- 

 posed of two principal classes of sub- 

 stance, nucleoproteins and a globular 

 tvpe of protein. Nucleoproteins are 

 composed of nucleic acids, associated 

 with protein. The protein associated 

 with nucleic acid seems to be of a 



CYTOLOGY 



relatively simple t)'pe— mostly histone. 

 Nucleic acids exist in the form of com- 

 plicated molecules, each composed of 

 units known as nucleotides: each nu- 

 cleotide consists of a purine or pyrim- 

 idinc base attached to a sugar, which 

 is in turn attached to a molecule of 

 phosphoric acid. The nucleotides are 

 arranged in parallel, somewhat like the 

 rungs of a ladder, which are attached 

 to each other, but only at one end. 

 The rung consists of the base and 

 sugar, the part which attaches the 

 rungs together at one end is the phos- 

 phoric acid. The latter also attaches 

 the nucleic acid to the associated pro- 

 tein. 



Considerable progress has been 

 made in recent years in the field of nu- 

 cleic acid chemistr}'. The work of 

 Watson and Crick, for instance, sug- 

 gests that nucleic acid molecules exist 

 in pairs, the two molecules wound 

 around one another, with the nucleo- 

 tides extending horizontally inward. 

 Each nucleotide is attached to a nu- 

 cleotide of the other nucelic acid mole- 

 cule by its base, in a ver^' precise man- 

 ner, the two nucleic acid molecules to- 

 gether thus resembling a twisted ladder. 

 It used to be thought that the nu- 

 cleotides containing the two purine 

 bases adenine and guanine and the 

 two pyrimidine bases thymine (or 

 uracil) and cytosine were present in a 

 nucleic acid molecule in equal num- 

 bers and arranged in a regular se- 

 quence. This is no longer found to be 

 true. Not only are the various nucleo- 

 tides present in varving proportions, 

 but there are also more than four kinds 

 of nucleotides now recognized. Far 

 from there being but a single pattern 

 of arrangement of four nucleotides, a 

 ver}' great variety of arrangements of an 

 unknown number of different nucleo- 

 tides is indicated. 



The question of first importance in 

 this study of chromosome chemistry is 

 what part of the chromosomal struc- 



