8 The Life Cycle of the Single Cell 



amoeboid masses of protoplasm that contain many nuclei. The proto- 

 plasm increases in amount and the nuclei divide as the organisms grow. 

 Each protoplasmic mass can fragment ( in the true sense ) to yield two or 

 more smaller masses. It should be noted that some higher animals can 

 also fragment into small pieces, each of which can then reconstitute a 

 whole new organism. Flatworms, starfish, and many others do this. 



Biochemical Events in the Cell Division Cycle 



In recent years, biologists have begun to study biochemical events 

 during the cell division cycle. Some have refined analytical techniques 

 to the point where they can study a single cell, weigh it ( despite the fact 

 that they must deal with weights of only millionths of a gram), and, by 

 microchemical analysis, determine the amounts of the cell constituents 

 (even though these are present in infinitesimal quantities). Other biolo- 

 gists have devised conditions to make cell populations divide synchro- 

 nously. Cell populations are usually asynchronous; that is, at any instant 

 some will be preparing to divide, some will have just completed division, 

 and others will be in an intermediate stage. With synchronized cells, one 

 need not apply very delicate and sensitive techniques for the study of a 

 single cell, but instead can work with large samples of cells and thereby 

 examine more easily the biochemical and morphological events that ac- 

 company each stage of the division cycle. This work has only begun, but 

 ultimately we may hope to understand how the cell coordinates the syn- 

 thesis of all its constituents. It is already clear from such studies that vari- 

 ous cell constituents are synthesized during very specific phases of the 

 division cycle. Figure 2 illustrates schematically the formation of deoxy- 

 ribose nucleic acid (DNA), the primary component of chromosomes and 

 the bearer of the cell's genetic constitution. As you can see, the DNA is 

 synthesized during a relatively short period between successive mitoses. 

 Contrary to what one might expect, no DNA is formed during the period 

 when one sees the chromosomes actually divide. 



NUCLEAR DIVISION 



The primary method of vegetative (as opposed to sexual) nuclear 

 division is that of mitosis. The particular stages need not concern us here. 

 The significance of mitosis is that it is an almost foolproof way of ensuring 

 that each daughter nucleus will possess a copy of each chromosome and, 

 therefore, a complete set of the genetic material contained therein. 



However, not all nuclei divide by mitosis. In some cells the nucleus 

 does not assume the mitotic configuration, but merely stretches out into a 



