Protoplasm, the Cell, and the Organism - 23 



genie materials. They mediate the transmis- 

 sion of heritable characteristics from genera- 

 tion to generation in the organism (Chap. 

 26). Each DNA unit possesses a remarkable 

 capacity for determining its own synthesis, 

 thus achieving se//-replication (p. 522). More- 

 over each carries a code of instructions for the 

 synthesis of other important components in 

 the cell (Chap. 27). 



At the time of cell division (Chap. 3), the 

 elongate DNA-protein threads become tightly 

 compacted into short rodlike bodies that are 

 easy to recognize as chromosomes (Figs. 2-1 

 and 2-11 A). By this time each thread has 

 undergone replication, forming duplicate sis- 

 ter chromosomes, which still lie side by side 

 (Fig. 2-11 A). Then, as cell division proceeds, 

 the duplicate chromosomes become separated 

 from each other. Thus finally, when two new 

 daughter cells have been formed, each re- 

 ceives one member of every pair of sister 

 chromosomes. By this means the chromoso- 

 mal and genie structures of the cell are per- 

 petuated from generation to generation. 



Between divisions the chromosomes ener- 

 getically carry out their synthetic activities. 

 The chromosomes become exceedingly elon- 

 gate, each appearing as a delicate, complexly 

 folded thread (Fig. 2-1 IB). In the uncoiled, 

 elongate form, the chromosomes are more 

 difficult to resolve. Moreover, since a number 

 of such elongate threads are massed together 

 within the confines of the nuclear membrane, 

 it is very difficult to distinguish them indi- 

 vidually. 



DNA proteins occur almost exclusively as 

 components of nuclei, or more particularly 

 of chromosomes (except in the case of 

 viruses). But RNA proteins (p. 525) are found 

 both inside the nucleus and outside, in the 

 surrounding cytoplasm. Both types of nucleo- 

 proteins display a generally similar chemical 

 structure (p. 525). However, they can be dis- 

 tinguished on the basis of a very useful stain- 

 ing process, called the Feulgen reaction. DNA 

 compounds, by virtue of their content of 

 deoxyribose sugar (p. 80), yield a positive 

 Feulgen coloration; whereas RNA com- 



pounds, in which the sugar constituent is 

 ribose, are negative to the Feulgen reaction. 



A considerable body of evidence indicates 

 that the RNA components of the cell originate 

 from the DNA elements. In fact DNA ap- 

 pears to provide a template (p. 134) for the 

 synthesis of RNA. Temporarily, RNA may 

 accumulate in or around a definitive center, 

 the nucleolus, but eventually much of it is 

 translocated into the cytoplasm (p. 526). The 

 importance of RNA in relation to the syn- 

 thesis of enzymes and other proteins in the 

 cell will be considered later (p. 134). 



Nucleoli are now generally recognized as 

 definitive intranuclear organelles. Generally, 

 nucleoli are Feulgen negative; the number 

 present is fixed and definite in each kind of 

 cell; they tend to disappear during each cell 

 division; and new nucleoli originate at fixed 

 points on specific chromosomes, after cell 

 division has occurred. 



Nuclei vary widely as to size, form, and 

 position. Usually, however, the nucleus is 

 rounded, and usually it lies near the center 

 of the cell. The relative size of the nucleus 

 also varies, from a small fraction to almost 

 the whole cell volume. In any one kind of 

 cell, however, the ratio between nuclear and 

 cytoplasmic volumes appears to remain quite 

 constant. 



The Cytoplasm. All the protoplasm of a 

 cell, exclusive of the nucleus, is the cytoplasm. 

 Under a light microscope, the main part of 

 the cytoplasm, which has been termed the 

 clear cytoplasmic matrix, gives the appear- 

 ance of an optically empty fluid, in which a 

 variety of visible bodies — mitochondria (p. 

 28), vacuoles (p. 28), fibrils (p. 30), etc.— are 

 suspended. 



The term clear cytoplasmic matrix, al- 

 though much used prior to the development 

 of electron microscopy, is nonetheless mis- 

 leading. The electron microscope shows that 

 the so-called clear cytoplasm possesses a com- 

 plex ultramicroscopic structure. This consists 

 mainly of an elaborate system of exceedingly 

 delicate interconnected double membranes, 

 collectively called the endoplasmic reticulum 



