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INTRODUCTION
The genetic information underlying plant form and function is contained within the cell nucleus in the form of complex, dynamic DNA-protein structures-the chromosomes. The faithful transmission of these structures during mitosis and meiosis requires extensive chromosome reorganization and the formation of a unique, transient structure, the spindle, which serves to segregate chromosomes to daughter cells. Recent studies indicate that chromosomes can undergo dynamic structural changes and movement within the nuclei of nondividing cells as well.
The biology of the nucleus, its contents and functions, is a complex, rapidly evolving topic (Lamond and Earnshaw, 1998). This review focuses on chromosome organization within the nucleus and during cell division. Communication between the nucleus and the cytoplasm, as mediated by nuclear pores, recently has been reviewed by Heese-Peck and Raikhel (1998). For more information on nuclear envelope structure and formation, see the review by Gant and Wilson (1997).
CHROMATIN ORGANIZATION WITHIN THE NUCLEUS
The interphase nucleus is compartmentalized, with the most conspicuous compartment being the nucleolus, the site of ribosomal RNA synthesis and ribosome assembly (reviewed in Shaw and Jordan, 1995). The remainder of the nucleus is filled with chromatin and the machinery required for its function, including the transcription and DNA replication machinery. When visualized by indirect immunofluorescence, many nuclear components, such as the ribonucleoprotein complexes involved in removing introns from nuclear mRNA precursors, localize not only as discrete bodies but also diffusely throughout the nucleoplasm (Lamond and Earnshaw, 1998). Individual chromosomes, however, cannot be discriminated within the mass of chromatin by conventional DNA staining techniques. Only with the recent development of fluorescence in situ hybridization (FISH) technology have individual chromosomes been discernible as nonoverlapping cloudlike structures (Lichter et al., 1988).
Euchromatin
By visualizing sites of BrUTP incorporation in rye nuclei, Abranches et al. (1998) have shown that there are multiple sites of transcription throughout a given chromosome cloud. The transcribed chromatin regions are associated with euchromatin (diffuse chromatin), whereas the heterochromatin (condensed chromatin) is not transcribed. Furthermore, transcriptionally active euchromatin is hypersensitive to nucleases (see Paul and Ferl, 1998a, and references therein), and it has been found to contain acetylated histones (reviewed in Pollard and Peterson,1998).
This latter observation has opened up an exciting area of current research with the discovery that some transcription factors...