It has long been known that the nuclear boundary either disappears 

 or undergoes major reorganization during cell division, and that a new 

 boundary is formed around each of the two daughter nuclei. Since the 

 structures concerned are well below the resolving power of the light 

 microscope it has been only recently, with the aid of electron micros- 

 copy, that information has been obtained concerning the structures and 

 sequence of events involved in re-establishment of the nuclear envelope 

 at cell division. Recent work has indicated that during mitotic prophase 

 the nuclear envelope breaks up into a number of separate vesicles which 

 become dispersed in the cytoplasm and are indistinguishable from the 

 membrane elements of the endoplasmic reticulum (Figure 4-6). 



The new envelope is formed at the end of the mitotic process around 

 each of the daughter nuclei. This involves migration of membrane ele- 

 ments of the endoplasmic reticulum onto the surface of the chromosomes 

 where they eventually coalesce and become reorganized into a double- 

 layered membrane structure (Figure 4-7). The selection of membrane 

 elements in formation of the nuclear envelope appears to be quite 

 random and not to involve necessarily those which arose during the 

 breakdown process in the previous prophase stage. 



Nucleoplasm 



This component of the nucleus appears in electron micrographs as 

 consisting of a pattern of irregular-shaped particles or granules. Little 

 is known regarding its chemical makeup. However, ultracentrifugation 

 studies indicate that it is essentially protein in character. It contains some 

 RNA and gives positive cytochemical reactions for glycoproteins. A 

 number of hydrolytic enzymes (i.e., ribonuclease, alkaline phosphatase, 

 dipeptidase) occur in the nucleus and may be specific constituents of 

 the nucleoplasm (Brachet, 1957). 



Nucleolus 

 Depending on the species, interphase nuclei may contain one or more 

 nucleoli and the number may be constant or variable (Figure 4-8 (a)). 

 Present evidence indicates that in the somatic and reproductive cells of 

 many organisms, both plant and animal, nucleoli are formed during the 

 telophase of mitosis in association with specific regions of specific chro- 

 mosomes of the complement. For the sake of convenience, such chromo- 

 somes are referred to simply as nucleolar chromosomes to distinguish 

 them from other members of the complement not displaying this activity 

 (Figure 4-8 (b)). A number of diploid species of plants have a single 

 pair of nucleolar chromosomes, in which case the nucleolus organized 



STRUCTURE AND FUNCTION OF THE NUCLEUS / 81 



