IV 



Cell Division 



At this point it is necessary to introduce another great family of 

 substances, which are important constituents of all living cells, viz. 

 the nucleic acids. They are so called because they were first found in 

 the cell nucleus — a specialized part of many cells which contains 

 thread-like bodies called chromosomes. They were given this name 

 because they can be stained by certain dyes and made visible. How- 

 ever, in recent years optical techniques have been developed which 

 enable the chromosomes to be photographed in living cells (see 

 Plate 2). 



There is no doubt that the chromosomes are intimately concerned 

 with the processes of cell division, whereby a single cell becomes 

 two cells, because they undergo changes during cell division which 

 suggest that they either control the whole process or are an essential 

 part of it. 



Just before the cell is ready to divide, the contents of the nucleus 

 can be seen, rather indistinctly, as a tangle of fine threads. They 

 become thicker and more definite and finally collect themselves into 

 distinct chromosomes. At a later stage in the processes (see Plate 3), 

 each chromosome is seen to be split into two threads more or less 

 parallel with each other. The members of each pair move away from 

 each other and finally collect together at opposite sides of the cell. 



In this process, the original set of chromosomes is duplicated, so 

 that when the cell divides into two, giving two daughter cells, each 

 contains a complete set of chromosomes like the original cell. It is 

 not surprising that cytologists see in this process the basic mechanism 

 of cell division, which ensures that each new cell has in it a full set of 

 the chromosomes. Later workers on genetics were able to correlate 

 the behaviour of the chromosomes with the laws of heredity, so that 

 they regarded the chromosomes as the carriers of the unit hereditary 

 characters called genes. The genes were believed to be arranged in a 

 linear order along the chromosomes and in some cases (particularly 



