THE CELL AS A FUNDAMENTAL UNIT OF LIFE 25 



in a linear order on long slender strings known as chromosomes. These 

 chromosomes form a mass known as the chromatin network which can 

 be seen in cells which are not in some stage of mitosis. In a typical 

 body cell of man there are 48 of these chromosomes, so there will be an 

 average of about 400 genes to each chromosome. It is quite evident 

 from this discussion that genes must be extremely small in size — so 

 small, in fact, that they cannot be seen by ordinary microscopic meth- 

 ods. Through various techniques, however, it has been established that 

 genes are probably very large molecules made up of nucleic acid com- 

 bined with protein to form nucleoprotein. Genes have the remarkable 

 power to take the materials absorbed by the cells as food and convert 

 them into other genes like themselves. Genes normally perform this 

 duplication only once for each cell division, so that the number of 

 genes remains constant throughout repeated cell divisions. This gene 

 duplication, then, is the beginning point for a study of cell duplication. 

 For purposes of convenience of study, the process of mitosis is usually 

 divided into distinct stages or phases which we will consider at this 

 time. 



Interphase. This is not actually a phase of mitosis, but a stage 

 in between mitoses. It is sometimes called the resting stage, but the 

 cell is resting only with respect to mitosis. It may actually be at its 

 peak of metabolism and growing rapidly. In typical cells at this stage 

 the chromosomes are extended to their greatest length and are often 

 so thin that they are not easily visible. Instead they may appear as a 

 diffuse chromatin network within the nucleus. 



Prophase, the phase of preparation. Typical cell duplication begins 

 with the duplication of the genes together with the threads upon which 

 they lie. Each chromosome is now in reality a double thread of genes, 

 but they adhere closely together and their dual nature is not readily ap- 

 parent in all cells. Each chromosome bears a body somewhere along 

 its length known as a centromere which is to play an important part 

 in mitosis. The centromeres do not become double when the genes 

 become duplicated, and thus each double gene string has only one 

 centromere at this stage. Next, the chromosomes gradually become 

 shorter and thicker so that they become clearly visible within the nu- 

 cleus. This shortening and thickening is accomplished by a coiling 

 of the chromosomes followed by a deposition of a matrix around the 

 coils as shown in Fig. 2.5. The matrix may obscure the coiled nature 

 of the strings of genes within the chromosome. It is often possible 

 to see the dual nature of the chromosomes, however. Each half of a 

 chromosome is known as a chromatid. 



