Chapter *3 

 MITOSIS 



I: 



"n looking for the biological basis 

 for the transmission of genetic 

 -material from parents to progeny 

 your attention has been called (p. 5) to the 

 cellular bridge between generations. Re- 

 member that it is only via this bridge that 

 genie transmission may take place, at least 

 in single-celled organisms for whom cell divi- 

 sion is equivalent to reproduction. More- 

 over, all cellular organisms are remarkably 

 similar in the way that they accomplish cell 

 division. To initiate our present search for 

 the material basis of genes let us examine 

 briefly certain general features of cell structure 

 and the appearance of cells undergoing 

 division, as seen under the microscope. 



There are two major parts of the cell, a 

 peripheral portion called the cytosome, con- 

 taining substances making 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 the formation of a cell plate, 

 which starts internally and proceeds toward 

 the periphery until the separation into two 

 daughter cells is complete. In the case of 

 animal cells, a furrow starts peripherally and 

 proceeds inward, until the parent cell is 

 cleaved into two. The degree to which the 

 two daughter cells are identical with re- 

 spect to cytoplasmic components 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, so that the 

 two daughter cells contain very different 

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amounts of cytoplasm. Although cytoplas- 

 mic components often may be distributed 

 unequally between daughter cells, this is not 

 true for the nuclear contents. Nuclear divi- 

 sion usually directly precedes cytosomal 

 division. But the nucleus does not simply 

 separate into two parts by forming a furrow 

 or cell plate. Instead of simply and directly 

 separating into two parts, the nucleus un- 

 dergoes a remarkable series of preparatory 

 activities before it divides; this process of 

 indirect nuclear division is called mitosis. 



Even though a nucleus shows no visible 

 evidence that it is going to undergo mitosis, 

 it is known to be very active chemically. In 

 appearance (Figure 3-1 A), it is bounded by a 

 nuclear membrane and is filled by a more or 

 less homogeneous-appearing ground sub- 

 stance in which one or more small bodies 

 called nucleoli are located. 



The first indication that the nucleus is 

 going to divide is the appearance in its ground 

 substance of a mass of separate fibers (Figure 

 3-1 B), some of which seem to be associated 

 with the nucleoli. These fibers are called 

 chromosomes. This appearance marks the 

 start of the first phase of mitosis, or prophase. 

 Careful cytological observation reveals that 

 each chromosome is in turn composed of two 

 delicate threads irregularly coiled about each 

 other. Each of the paired threads within a 

 chromosome is called a chromatid. As pro- 

 phase continues the chromatids within each 

 chromosome become shorter and thicker and 

 untwist from each other (Figure 3-lC). The 

 nucleoli become smaller and it is believed 

 that some of the material incorporated to 

 thicken the chromatids is derived from the 

 nucleoli. By the end of prophase (Figure 

 3-1 D) the nucleoh and nuclear membrane 

 have disappeared and the chromatids have 

 formed thick rods which begin to move ac- 

 tively for the first time. Active motility is 

 not the property of the entire chromosome, 

 but is restricted to a particular region of it 

 called the centromere. 



