24 



THE CELL AS A FUNDAMENTAL UNIT OF LIFE 



you these characteristics. There are so many functions to be per- 

 formed by the genes that there must be a very large number of them — - 

 a recent estimate puts this number at about 20,000 for each human body 

 cell. Yet, if cell division is to be true cell duplication, each of these 



Centrosome 



Chromatin 

 network 



Spindle figure 

 Chromosome 



Fig. 2.3. Mitosis in a typical animal cell. These diagrams show the stages of cell 

 duplication as seen in the usual stained specimens. A. Interphase. B. Early prophase. 

 C. Late prophase. D. Metaphase. E. Anaphase. F. Early telophase. G. Late telo- 

 phase. H. Daughter cells. 



genes must first become duplicated and one of each kind of gene passed 

 to each of the new cells formed. 



Such a process appears to be a major engineering undertaking on 

 the part of the cell, and it certainly is ; but it is greatly simplified by 

 the method of gene arrangement within the nucleus. The genes are 

 not indiscriminately scattered throughout the nucleus, but are arranged 



