Sister kinetochores

Loss of sister chromatid cohesion would therefore be sufficient for the sudden movement of chromatids to opposite poles at the metaphase to anaphase transition. According to this hypothesis, a specific apparatus binds chromatids together during replication, holds them in an orientation that facilitates the attachment of sister kinetochores to spindles extending to opposite poles, and resists the splitting force that results from this bipolar attachment to the spindle. Destruction of this specialized cohesive structure triggers movement of chromatids to opposite poles at the onset of anaphase. 

Goshima G, Yanagida M 2000 Establishing biorientation occurs with precocious separation of the sister kinetochores, but not the arms, in the early spindle of budding yeast. Cell 100 619-633... 

Lehner There is a paper by Goldstein (1981) in which he describes electron microscope (EM) studies of meiotic chromosomes. At meiosis I, chromosomes do not have two sister kinetochores. There is a maturation into two kinetochores between the two meiotic divisions. Have comparable EM analyses been done in yeast ... 

When both sister kinetochores are attached correctly, they are pulled in opposite directions by the microtubule fibers and the centromere is under tension (Fig. 3a). In this model, the absence of centromere tension would keep the checkpoint active. Small molecules that target tubulin have provided a way to test these models experimentally. Nocodazole depolymerizes microtubules, which creates unattached kinetochores (Fig. 3c), whereas taxol stabilizes microtubules but inhibits their dynamics, which decreases the centromere tension (Fig. 3b) (29). 

Rieder CL, Schultz A, Cole R, Sluder G. Anaphase onset in vertebrate somatic cells is controlled by a checkpoint that monitors sister kinetochore attachment to the spindle. J. Cell. Biol. 1994 127 1301-1310. 

Fig. 16. Electron micrograph of a blood lily Haemanthus katherinae) chromosome in early mitotic prometaphase. Unipolar malorientation to a pole toward the left is suggested by the chromosomal microtubule arrangement at the two sister kinetochores ("ki/ The overall spindle axis is indicated by the arrow at the lower left side. The circle is a stain mark. XI 2,000. (From Bajer and Mole -Bajer. 1969. Chromosoma, 27 448-484.)...

Granted that the proposed explanation is at least reasonable for both meiotic and mitotic chromosome distribution, can the fundamental difference between these divisions also be explained As Ostergren (1951) proposed, a kinetochore structural difference may parallel the kinetochore orientation difference. An opposite polarity of sister kinetochores in mitosis has just been invoked to explain, in part, achievement of bipolar amphiorientation. On the same assumptions, if sister kinetochores lie close together and have a common polarity, their orientation to the same pole (syntely) should at least be facilitated. The required polarity occurs in the one example studied at late prophase (Amphiuma, p. 267). 

Fig. 17. Electron micrograph of a Chinese hamster fibroblast in prophase. Portions of two chromosomes enclosed within the nuclear envelope are shown in one the two sister kinetochores I ki/ "k2") are visible note their "back-to-back" arrangement. X60,000. The material was not treated with colcemid Icf. Brinkley and Stubblefield, 1966) glutaraldehyde fixation, post-fixed in 0s04, uranyl and lead staining. (Unpublished micrograph courtesy of B. R. Brinkley.)...

Exactly this is observed in studies on fixed cells (reviewed by John and Lewis, 1965, pp. 52ff.) and following experimental destruction of the linkage by micromanipulation of living cells (Nicklas, unpublished). A more exact prediction is possible from the postulated role of bipolar tension in reorientation unpaired meiotic chromosomes that retain the meiotic synorientation of both sister kinetochores to the same pole should be unstable and frequently reorient. If, however, they achieve amphiorientation, as do mitotic chromosomes, a stable bipolar orientation should result. Both conditions occur in nature and with the expected result (living cells with unpaired sex chromosomes, Dietz, 1956 Bauer et al., 1961 Nicklas, 1961 fixed cells, reviewed by John and Lewis, 1965). 

In vitro in human cells, induction of DNA strand breaks was shown to be dependent on the presence of Cu(II). Hydroquinone induced sister chromatid exchanges and chromosomal aberrations without an exogenous metabolic system. The metabolic activation system was not required for the induction of micronuclei in human lymphocytes where kinetochore-positive micronuclei were found. 

Three types of microtubule can readily be defined in the mitotic spindle. Polar microtubules overlap (and probably interact) between the poles and are involved in pushing the poles apart in anaphase. Astral microtubules radiate in all directions and also help separate the poles. Kinetochore microtubules attach themselves to specialized protein structures (kinetochores) located on each side of the centromere of each chromosome. These microtubules are involved in moving the chromosomes to the metaphase plate and in separating sister chromatids at anaphase. The microtubules in the spindle are very dynamic and have a half-life of only a few seconds. This appears to be especially important in the capture of chromosomes by the kinetochore microtubules. Microtubules that miss the target kinetochores are quickly lost because their dynamic instability soon leads to depolymerization. The new microtubules that form may hit the target and be partially stabilized through plus-end capping. 

During anaphase, when the sister chromatids split at the centromere, this balance is lost [Fig. 5-37(/>)J. The dynein motors in the kinetochores drive the sister chromatids to the poles. The speed at which this happens may be regulated by the controlled depolymerization of the kinetochore microtubules. The poles can now move apart and the overlap between polar microtubules shortens. It would be completely lost but for rapid polymerization at the plus ends of these filaments. 

Centromere is that site on the chromosome that is attached to the mitotic spindle during divison of the nucleus in mitosis. The centromere is also the point in the condensed chromosome where the two sister chromatids are connected. In the late prophase, the centromere has two kinetochores, one for each sister chromatid. 

Figure 4 Correction of improper chromosome attachments by activation of Aurora kinase (45). (a) Assay schematic, (i) Treatment with the Eg5 inhibitor monastrol arrests cells in mitosis with monopolar spindles, in which sister chromosomes often are both attached to the single spindle pole, (ii) Hesperadin, an Aurora kinase inhibitor, is added as monastrol is removed. As the spindle bipolarizes with Aurora kinase inhibited, attachment errors fail to correct so that some sister chromosomes are still attached to the same pole of the bipolar spindle, (iii) Removal of hesperadin activates Aurora kinase. Incorrect attachments are destabilized by disassembling the microtubule fibers, which pulls the chromosomes to the pole, whereas correct attachments are stable, (iv) Chromosomes move from the pole to the center of the spindle as correct attachments form, (b) Structures of the Eg5 inhibitor monastrol and two Aurora kinase inhibitors, hesperadin and AKI-1. (c) Spindles were fixed after bipolarization either in the absence (i) or presence (ii) of an Aurora kinase inhibitor. Arrows indicate sister chromosomes that are both attached to the same spindle pole. Projections of multiple image planes are shown, with optical sections of boxed regions (1 and 2) to highlight attachment errors. Scale bars 5 xm. (d) After the removal of hesperadin, GFP-tubulin (top) and chromosomes (bottom) were imaged live by three-dimensional confocal fluorescence microcopy and DIC, respectively. Arrow and arrowhead show two chromosomes that move to the spindle pole (marked by circle in DIC images) as the associated kinetochore-microtubule fibers shorten and that then move to the center of the spindle. Time (minutes seconds) after the removal of hesperadin. Scale bar 5 (cm.

As prophase continues, a set of microtubules grows from the kinetochore of each sister chromatid. The microtubules extending from each chromatid become attached to opposite poles of the spindle. 

In higher eukaryotes, a complex protein structure called the kinetochore assembles at centromeres and associates with multiple mitotic spindle fibers during mitosis. Homologs of most of the centromeric proteins found in the yeasts occur in humans and other higher eukaryotes and are thought to be components of kinetochores. The role of the centromere and proteins that bind to it in the segregation of sister chromatids during mitosis is described in Chapters 20 and 21. 

As mentioned earlier, each sister chromatid of a metaphase chromosome Is attached to microtubules via Its kinetochore, a complex of proteins assembled at the centromere. The opposite ends of these kinetochore microtubules associate with one of the spindle poles (see Figure... 

At metaphase, the spindle Is In a state of tension with forces pulling the two kinetochores toward the opposite spindle poles balanced by forces pushing the spindle poles apart. Sister chromatids do not separate because they are... 

Nondisjunction. This abnormality occurs when chromosomes segregate in anaphase before the kinetochore of each sister chromatid has attached to microtubules (red lines) from the opposite spindle poles. As a result, one daughter cell contains two copies of one chromosome, while the other daughter cell lacks that chromosome. [Adapted from A. Murray and I Hunt, 1993, The Cell Cycle An Introduction, W. H. Freeman and Company]... 

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