MYSTERIES OF LIFE AND EXISTENCE — SNODGRASS 523 



some equipment of the species. Sexual reproduction, therefore, is 

 dependent on chromosome reduction in the conjugating cells, but the 

 same thing takes place in every dividing cell. In ordinary cell divi- 

 sion, however, the chromosomes are first doubled. 



When we turn from the gross structure of an animal to what goes 

 on w^ithin its cells, we leave the realm of anatomy for that of bio- 

 chemistry. Here we find ourselves in a totally different world of life, 

 in fact, in that of life itself, and we encounter phenomena quite dif- 

 ferent from anything we have experienced before. 



The mechanism of cell division in the Metazoa is highly developed 

 in a form known as mitosis (see Mazia, 1953) . In the resting cell the 

 nuclear chromosomes are long, tangled filaments, but preceding 

 mitosis they condense into darkly staining bodies usually of a definite 

 number in each species. Prior to this the chromosomes have divided 

 each into two, so that now they are present in identical pairs. A 

 small body in the cytoplasm, known as the centriole, then divides and 

 the two parts move to opposite poles of the cell. Threadlike fibers 

 radiate out from each centriole, forming a spindle through the nucleus, 

 which now accommodatingly loses its wall. Some of the threads 

 extend from pole to pole, others attach to individual chromosomes. 

 The two chromosomes of each pair then move to opposite poles as if 

 pulled by the threads, but the mechanism of their movement is not 

 fully understood. Each chromosome group becomes a new nucleus. 

 Finally the cell constricts at the equator between the nuclei and even- 

 tually divides into two duplicate cells. 



All this coordinated activity observed in the dividing cell appears 

 to take place automatically. It can be followed under the microscope, 

 but there is no visible evidence of what causes the phenomena seen. 



The biochemists have shown that the principal substance of the 

 chromosomes in all animals is a chemical laiown as deoxyribonucleic 

 acid, of highly complex molecules. This acid, called DNA, is truly a 

 most remarkable stuff; it is the "dictator" of all the cell activities, 

 including cell metabolism and the growth of the embryo developed 

 from the Qgg. Intensive studies of these subjects have been made by 

 the biochemists in recent years. A dramatic example of the creative 

 power of DNA is seen in the infection of a bacterium by a virus. The 

 virus cell discharges its DNA into the bacterium, and here, at the 

 expense of the bacterial cytoplasm, the DNA molecules not only 

 replicate themselves but form complete new virus cells, as many as 

 200, which are discharged with the rupture of the bacterium (Jacob 

 and Wollman, 1961). 



The cytoplasmic changes that differentiate the multiplying cells are 

 said to be done strictly "on orders" from the DNA of the nucleus, and 

 are carried to the cytoplasm by a related substance RNA (ribonucleic 

 acid) catalyzed by enzymes from the DNA. In connection with 



