Recent studies showed that an oligomeric array of Ndc80 complexes move along the microtubules through biased diffusion Powers et al. In this way, microtubules are able to attach to kinetochores while at the same time sustaining the loss or gain of tubulin subunits at the microtubule plus ends. The interaction between the Ndc80 complex and microtubules is regulated by Aurora B kinase, which phosphorylates Ndc80 and reduces its binding affinity in vitro Cheeseman et al. A recent study reported that the physical distance between Aurora B kinase and Ndc80 determines whether Ndc80 is phosphorylated or not Liu et al.
Since Aurora kinase tends to be localized in the inner kinetochore area, correctly attached, stretched, and bi-oriented outer-kinetochores are separated from the kinase and retain their tight association with microtubules Liu et al.
On improperly aligned kinetochores, Aurora B kinase and Ndc80 are in close proximity, causing the microtubule attachments to be unstable until proper attachments can be made. This mechanism is entirely consistent with the available limited literature on plants.
Aurora kinases are present in plants and bind to kinetochores Demidov et al. Other components of the Ndc80 complex can also be detected in plants by sequence similarity Meraldi et al. The emerging view is that Ndc80, as regulated by selective phosphorylation of key outer residues, is sufficient to organize and maintain stable contacts with spindle microtubules Akiyoshi et al.
The primary arguments against the idea that kinetochores are involved in spindle formation derive from fact that spindles can form in Xenopus extracts that lack centromeres. However, it is important to note that while bipolar spindles do self-assemble in such extracts, it has never been shown that the DNA-coated beads align to a metaphase plate or move in an anaphase fashion.
We believe that the Xenopus results may be more reflective of the role of chromosome arms, which in both animals and plants are known to bind microtubules and facilitate chromosome bi-orientation Khodjakov et al.
This view is supported by a recent study of human cells where the authors showed that when the Ndc80 complex protein Nuf2 was knocked down, the balance of microtubules shifted towards chromosome arms O'Connell et al.
One interpretation is that the Ran cycle naturally promotes the formation of microtubules around chromatin early in the cell cycle when the kinetochores are underdeveloped.
As the cell cycle proceeds and kinetochores mature, the kinetochores become the strongly preferred sites of microtubule attachment. The elegance of mitosis has caught the imagination of scientists for over a century and spawned many models to explain it Mitchison and Salmon The current view is more of a hybrid among these concepts, presuming that multiple forces operate at different times and with varying importance depending on the species Gadde and Heald ; Wadsworth and Khodjakov Microtubules form around chromosomes, kinetochores, and spindle poles such that they interact and converge into the mature spindle.
Here, we argue that the same basic principles of spindle formation apply to plants but with the addition of the preprophase band that facilitates the orientation of spindles Lloyd and Chan ; Bannigan et al.
We emphasize that this perspective is necessarily based on a very small plant literature and relies heavily on data from animals. Given these caveats, we can infer that plant spindle formation is initiated by a RanGTP gradient that favors the formation of microtubules around chromosomes following nuclear envelope breakdown.
We presume that the placement and orientation of the spindle is influenced by the presence of bridging microtubules derived from the preprophase band. The newly stabilized microtubules, already adopting a bipolar orientation, are then further oriented by the paired kinetochores, which take the primary role in stabilizing and reinforcing a strictly bipolar arrangement. Kinetochores promote microtubule nucleation and form kinetochore fibers by stabilizing the plus ends while the minus ends aggregate into loose poles by natural bundling factors such as kinesins Bannigan et al.
A comparison of spindle formation in animals and plants can be viewed in Fig. A comparison of spindle formation in animals and plants. Centrosomal and PPB-associated microtubules are shown in red , while kinetochore-associated microtubules are shown in green. Chromatin is shown in blue and kinetochores in yellow. The PPB is a ring that encircles the cell and the great majority of microtubules lie within the band.
A small minority of microtubules bridging microtubules, shown with dotted lines extend from the band to the nucleus. Unfortunately, the molecular mechanism of these processes is still largely unknown.
There are multiple basic observations still missing from the plant literature. These studies are difficult and can be viewed as confirmatory but are nevertheless required for further progress. Other more fundamental limitations are that an in vitro plant-based model of the same power as Xenopus has yet to emerge and that the chromosomes and spindles of Arabidopsis are nearly too small to be useful for mitosis research.
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Correspondence to R. Kelly Dawe. Reprints and Permissions. Zhang, H. Mechanisms of plant spindle formation. Chromosome Res 19, — Download citation. Published : 19 March Issue Date : April Anyone you share the following link with will be able to read this content:.
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Skip to main content. Search SpringerLink Search. Download PDF. Abstract In eukaryotes, the formation of a bipolar spindle is necessary for the equal segregation of chromosomes to daughter cells. Introduction Chromosome segregation relies entirely on the formation of a stable, bipolar spindle that can direct the separated chromosomes away from each other and into daughter nuclei.
Preprophase band: an analog of centrosomes? Full size image. Kinetochores: a re-evaluation There is little doubt that kinetochores are integral components of intact spindles and that they are required for chromosome movement.
Summary and discussion The elegance of mitosis has caught the imagination of scientists for over a century and spawned many models to explain it Mitchison and Salmon Microtubules that form the spindle fibers come from centrosome s, which are organelles located in opposite poles near the nucleus.
In mitosis, these filament s form at opposite poles of the cell and meet at the equatorial plane. Collectively, they form a spindle-shaped structure, which is widest at the middle then tapers at both ends. The spindle fibers form during prophase. During metaphase of cell division , the spindle fibers radiate from the centrioles at the opposite poles.
Some of them attach to the kinetochore s of the chromosome s while others bind to the arms of the chromosome s, still others continue to grow. When the spindle fibers start to pull the chromosome s via their kinetochores to opposite poles, this marks the anaphase of cell division.
Variant: spindle fibre See also:. Muscle cells are specialized to generate force and movement. Learn about the different types of muscle tissues in this tutorial and the molecular mechanisms of contraction
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