MITOSIS IN CELLS WITH UNREPLICATED GENOMES (MUGS) - SPINDLE ASSEMBLY AND BEHAVIOR OF CENTROMERE FRAGMENTS

Chinese hamster ovary (CHO) cells, which are arrested at the G1/S-phas e of the cell cycle with hydroxyurea, enter mitosis prematurely when t reated with caffeine [Schlegel and Pardee, 1986: Science 232:1264-1266 ]. Such mitotic cells with unreplicated genomes (MUGs) can assemble a mitotic spindle and progress through M-phase even in the absence of in tact, replicated chromosomes [Brinkley et al., 1988: Nature 336:251-25 4; Zinkowski et al., 1991: J. Cell Biol. 113:1091-1110; Christy et al. , 1995: Protoplasma 186:193-200]. In order to better define the role o f the spindle in chromosome movement, we compared the structure and as sembly of mitotic spindles and analyzed the nature of kinetochore asso ciation and movement in control cells and MUGs. The mitotic spindles i n MUGs display the same morphological features and dynamic properties of assembly-disassembly as seen in normal spindles. Although multiple centromere-kinetochore fragments (CKFs), derived from fragmented chrom osomes, interact with and attach to spindle microtubules in both ortho dox and unorthodox ways, they nevertheless become aligned on the metap hase plate. Prometaphase congression and alignment at metaphase is ach ieved in MUGs even though CKFs represent kinetochore fragments that or iginate from unreplicated chromosomes and, therefore, lack ''sister ki netochore'' orientation such as seen in chromosomes of control cells. Our study supports the notion that much of the ''information'' needed for prometaphase chromosome movement and alignment is endemic to the s pindle [Heald et al., 1996: Nature 382:420-425; Zhang and Nicklas, 199 6: Nature 382:466-468. (C) 1997 Wiley-Liss, Inc.