Units of Replication 27 



tween the cytoplasm and the nucleus, and the role of messenger 

 and transfer RNA in translating the DNA code into protein struc- 

 ture is carried on. The mechanism of this exchange is not under- 

 stood, however. Evidence clearly indicates movement of nucleic 

 acid from nucleus to cytoplasm. Actual particles usually ha\e not 

 been found in the pores of the nuclear envelope, as seen in the 

 electron microscope. Cytologists have reported that portions of the 

 nuclear envelope may pinch or bud off into the cytoplasm, where 

 the pieces have the appearance of endoplasmic reticulum or mito- 

 chondria. 



CELL DIVISION: MITOSIS 



When cells divide (Fig. 2.2), the first conspicuous change usually 

 is in the appearance of the nucleus. The chromosomes become visible 

 within the nuclear envelope in living or stained cells and, usually 

 concomitantly, the nucleolus decreases in size. This first of the 

 arbitrarily designated stages of mitosis or nuclear division is pro- 

 phase. Toward the end of this stage, it can be seen in the cells of 

 many organisms that the chromosomes are double, each consisting 

 of two half chromosomes ( chromatids ) . In most organisms the dis- 

 appearance of the nuclear envelope marks the beginning of promet- 

 aphase. During this period, or somewhat prior to it, a spindle-shaped 

 bundle of fibers (now known to be microtubules) is organized in 

 the cytoplasm. Toward the end of prometaphase the chromosomes 

 become arranged in a group at the equator of this structure. 



The spindle in those organisms in which it can be isolated and 

 studied chemically is composed of fibrous protein molecules con- 

 taining many sulfhydryl linkages and apparently oriented by the 

 centrioles at either end. In animals ( and in some plants ) the centrioles 

 also are surrounded by a pompon of fibers, the aster. In the somatic 

 cells of most plants no asters or centrioles are visible, but they may 

 be conspicuous in the reproductive cells. During the brief stage 

 called metaphase, the chromosomes are arranged across the equator 

 of the spindle with at least their spindle attachment points or centro- 

 meres in essentially a plane at right angles to the long a.xis of the 

 spindle. Very shortly thereafter the centromeres appear to divide 

 (they may actually have split at an earlier period) and the chroma- 

 tids— now daughter chromosomes— move to the poles. 



The phase of chromosome movement is called anaphase, and its 

 mechanism is still not understood. None of the current theories satis- 

 factorily explains the behavior of chromosomes and cells in all organ- 

 isms. In some animals, for example, certain chromosomes behave 



