We have proposed previously a kinetochore motor-polar ejection model for chromosome congression to the metaphase plate where forces generated at the kinetochore are antagonized by away-from-the pole forces generated within each half-spindle on the chromosome arms. This model was based in large part on observations of the behavior of chromosomes on monopolar spindles. In these cells chromosomes typically become attached to the pole by only one kinetochore fiber. These mono-oriented chromosomes move to positions away from the pole even though they are pulled poleward at their kinetochores. Their arms are also ejected away from the pole when severed from the centromere. Here we have characterized further the properties of monopolar spindles in newt lung epithelial cells to determine the similarities between monopolar and bipolar spindles. We found no significant differences between monopolar and bipolar spindles over the parameters examined, which included: microtubule dynamics as measured by fluorescence redistribution after photobleaching; the ability of polar microtubule arrays to push chromosome arms away from the pole; the dependence of chromosome position relative to the pole on microtubule assembly; the number of kinetochore microtubules per kinetochore; and the directional instability of kinetochore motion during chromosome oscillations poleward and away-from-the-pole. As in bipolar spindles, kinetochore directional instability is characterized by abrupt switching between constant velocity phases of poleward and away-from-the-pole motion. From these data we conclude that the mechanism(s) responsible for chromosome positioning in monopolar spindles are fundamentally the same as those in bipolar spindles; only the geometry of the two spindle forms and the interplay between sister kinetochore directional instabilities are different. We also found no correlation in the kinetochore-to-pole distance with kinetochore microtubule number in monopolar spindles, but a strong qualitative correlation with microtubule density. This finding indicates that oscillations of mono-oriented chromosomes in both monopolar and bipolar spindles occur because chromosomes persist in poleward motion until they reach a density of polar microtubules sufficiently high to promote switching to away-from-the-pole motion. As the kinetochore and chromosome arms move away-from-the-pole, microtubule density decreases and the kinetochore switches to poleward motion, pulling the chromosome arms back into regions of higher microtubule density. The mechanism regulating kinetochore switching between poleward and away-from-the-pole motion is poorly understood, but may depend on tension at the kinetochore generated by pushing forces on the chromosome arms produced by the polar microtubule arrays.
Microtubule assembly and kinetochore directional instability in vertebrate monopolar spindles: implications for the mechanism of chromosome congression
L. Cassimeris, C.L. Rieder, E.D. Salmon; Microtubule assembly and kinetochore directional instability in vertebrate monopolar spindles: implications for the mechanism of chromosome congression. J Cell Sci 1 January 1994; 107 (1): 285–297. doi: https://doi.org/10.1242/jcs.107.1.285
Download citation file:
Advertisement
Cited by
Special Issue: Cell Biology of Lipids
-LipidSI.png?versionId=3647)
Lipids are the main component of cellular membranes, but have key roles in many essential cellular processes, as well as structural and signalling functions. Our 2022 special issue showcases the diversity of current research in lipid cell biology.
The special issue includes an interview with Guest Editor James Olzmann. James has also contributed to this lipid droplet wall poster from our sponsor, Cayman Chemical.
Propose a new Workshop
-GSWorkshop.png?versionId=3647)
Our Workshops bring together leading experts and early-career researchers from a range of scientific backgrounds. Applications are now open to propose Workshops for 2024, one of which will be held in a Global South country.
We’re hiring: Community Manager, preLights
-preLights.jpg?versionId=3647)
We are seeking to appoint a new Community Manager to run our preprint highlighting service, preLights. Launched in 2018, this initiative has gained significant attention from researchers and the publishing industry. We are now looking for the right person to join us for the next phase of community building and the site’s growth and development. Please see the full job description for further details.
The application deadline is 20 May 2022.
Essay series: Equity, diversity and inclusion in cell biology
-EssaySeries.png?versionId=3647)
The JCS Essay Series is an initiative to help showcase and provide a platform for voices in the field of cell biology. The first topic we covered was 'Equity, diversity and inclusion in cell biology', and the winning and runner up essays are now available to read.
FocalPlane Network launched
-FocalPlaneNetworkLaunch.png?versionId=3647)
We are excited to announce the launch of the FocalPlane Network, an international directory of microscopists. The idea behind the FocalPlane Network is to facilitate promotion and networking as well as assist those seeking conference speakers, committee members, reviewers or collaborators. We hope that it will help promote diversity in the community. Find out more and join the Network here.