Searching for the middle ground: mechanisms of chromosome alignment during mitosis

Chromosome alignment at the spindle equator, or congression, is a remarkably conspicuous event during mitosis that defines the metaphase stage of the cell cycle. This movement of chromosomes to the spindle equator is necessary for accurate segregation of a cell’s replicated DNA in organisms as diverse as plants, insects, and mammals (for review see Khodjakov et al., 1999). Results from more than a century of detailed observations of chromosomes in mitosis (particularly in vertebrate cells) have provided a well-scripted sequence for the steps involved in chromosome attachment to the spindle and subsequent congression to the spindle equator (for review see Rieder and Salmon, 1994). In addition, chromosome cutting experiments and microtubule marking experiments have revealed many, if not all, of the forces involved in driving chromosome movement (Gorbsky, 1992). However, a striking gap exists in our understanding of the mechanisms of chromosome movement due to our inability to identify specific molecules that drive chromosome movement or regulate chromosome alignment at the spindle equator. This review highlights recent results that begin to fill this gap and examines current models for chromosome congression in the context of this new data. Microtubule–chromosome interactions occur primarily at kinetochores, specialized pairs of disc-shaped structures located on either side of the centromere. To congress to the spindle equator, a chromosome must biorient, i.e., attach to spindle microtubules with each kinetochore interacting with microtubules derived from one of the two spindle poles. Some chromosomes biorient immediately upon nuclear envelope breakdown and oscillate about the spindle equator, but do not tend to stray far from the spindle midzone (Fig. 1, b–e). Other chromosomes initially interact with microtubules at only one kinetochore. This leads to rapid chromosome movement toward the pole as it slides along the length of the microtubule in a manner similar to the transport of vesicles (Fig. 1 b) (Rieder and Alexander, 1990). Once near the spindle pole the kinetochore captures multiple microtubule plus-ends and builds a kinetochore fiber (Fig. 1 c). These monooriented chromosomes are positioned with their kinetochores pulled toward the pole and their arms pushed away from the pole and oscillate toward and away from their attached pole. During these oscillations, changes in kinetochore fiber length coincide with chromosome movement toward and away from the spindle pole. Eventually, a microtubule from the opposite pole will contact the unattached sister kinetochore establishing biorientation. The newly bioriented chromosome then moves in a directed manner (i.e., congresses) to the spindle equator (Fig. 1, d and e). An appealing model for how chromosomes congress to the center of the spindle is based on ideas developed by Ostergren (1951). In this model, chromosomes attached to two spindle poles experience force toward each pole with the magnitude of each force being proportional to the length of the kinetochore fiber connecting the chromosome to the pole. Chromosomes align at the equator of the spindle where opposing poleward forces are equal and balanced. However, two key observations suggest that this model is not likely to be correct (for review see Rieder and Salmon, 1994). First, in many cell types chromosomes oscillate back and forth over substantial distances as they congress to the center of the spindle, indicating that forces acting on chromosomes do not change monotonically with distance from a spindle pole. Second, monooriented chromosomes display both poleward and away-from-the-pole movements, indicating that chromosomes experience force away from the pole independent of any attachment to the distal pole (the polar ejection force). These observations are more consistent with models that proposed “smart” kinetochores capable of integrating different signals and forces to determine their position in the spindle, as they dance toward its center (Mitchison, 1989a). Address correspondence to Tarun M. Kapoor, Laboratory of Chemistry and Cell Biology, The Rockefeller University, 1230 York Ave., New York, NY 10021. Tel.: (212) 327-8176. Fax: (212) 327-8177. E-mail: [email protected]