6 



CHAPTER 1 



to reproduction. All cellular organisms are 

 remarkably similar in the way they accom- 

 plish cell division. Accordingly, let us ex- 

 amine briefly certain general features of cell 

 structure and the appearance, under the 

 microscope, of cells undergoing division, in 

 initiating our search for the material basis 

 of the genotype. 



There are two major parts of the cell 

 (Figure 1-3): a peripheral portion compris- 

 ing the cytosome, containing substances mak- 

 ing up cytoplasm, and a more central portion 

 called the nucleus, containing nucleoplasm. 

 In the final stages of cell division in higher 

 plants, the cytoplasm is divided by a cell 

 plate, whose growth starts internally and 

 proceeds toward the periphery until the sep- 

 aration into two daughter cells is complete. 

 In the case of animal cells, a furrow starts 

 at the periphery of the cell and deepens until 

 the parent cell is cleaved into two. The 

 degree to which the two daughter cells are 

 identical with respect to cytoplasmic com- 

 ponents depends upon the position of the 

 cell plate or furrow in the parent cell. In 

 some cases these occur in the middle of the 

 cell, but in many other cases they are located 

 off-center, producing daughter cells which 

 contain very different amounts of cytoplasm. 

 Although the cytoplasmic components of a 

 parent cell are often distributed unequally 

 between daughter cells, this is not true for 

 the nuclear contents. Ordinarily, nuclear 

 division directly precedes cytosomal division. 

 But the nucleus does not simply separate 

 into two parts by the formation of a furrow 

 or cell plate. Instead, the nucleus under- 

 goes a remarkable series of activities in order 

 to divide; this whole process of indirect nu- 

 clear division is called mitosis. 



During the time that a nucleus shows no 

 visible evidence of mitosis, it is nevertheless 

 very active biochemically. In appearance 

 (Figure 1-4A), it is bounded by a nuclear 

 membrane and is filled by a more or less 



homogeneous-appearing ground substance or 

 matrix in which are located one or more 

 small bodies, called nucleoli. 



The first indication that the nucleus is pre- 

 paring to divide is the appearance in its 

 ground substance of a mass of separate fibers 

 (Figure 1-4B), some of which seem to be 

 associated with the nucleoli. These fibers 

 are called chromosomes, and their appear- 

 ance marks the start of the first phase of 

 mitosis, or prophase. Careful cytological 

 observation reveals that each chromosome 

 is in turn composed of two major delicate 

 threads irregularly coiled about each other. 

 Each of the paired threads within each chro- 

 mosome is called a chromatid. As prophase 

 continues, the chromatids within each chro- 

 mosome become shorter and thicker and 

 untwist from each other (Figure 1-4C). 

 Some of the material incorporated to thicken 

 the chromatids may be derived from the 

 nucleoli, which are seen to become smaller. 

 By the end of prophase (Figure 1-4D), the 

 nucleoli and nuclear membrane have disap- 

 peared and the chromatids have formed 

 thick rods which begin to move actively for 

 the first time. Active motility is not the 

 property of the entire chromosome, however, 

 but is restricted to a particular region of it 

 called the centromere or kinetochore (see 

 p. 379). 



The centromeres move in a particular di- 

 rection relative to a fibrillar structure called 

 the spindle which has been forming through- 

 out prophase. The completed spindle has 

 a shape similar to what is produced when 

 one extends and separates the fingers and 

 touches corresponding fingertips together. 

 The wrists represent the poles of the spindle 

 and the fingers, the spindle fibers. The chro- 

 mosomes migrate from whatever position in 

 the spindle region they may have, until each 

 centromere comes to lie in a single plane 

 perpendicular to the axis between the poles, 

 that is, at the equatorial plane or equator 



