Molecular basis of chromosome segregation

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fig 2 Kinetochores and checkpoint signalling

(a) Electron microscopy reveals distinct plates within the kinetochore, and immunoelectron microscopy has localized molecules to the regions indicated. The dynamic plus ends of kinetochore microtubules interact with the outer corona and outer plate. (b) There is a dynamic exchange of checkpoint proteins at unattached kinetochores. Mad2, and other checkpoint proteins, are recruited from soluble pools and interact with a scaffold, of which Bub1 and Mad1 could form an important part. The checkpoint proteins are then released from kinetochores, perhaps in a quaternary complex (Mad2-Cdc20-BubR1-Bub3), to act as an APC inhibitor. (c) Bipolar attachment of sister kinetochores leads to stretching of the centromeric DNA and tension. How this is monitored remains unclear but the Ipl1 (Aurora) kinase has been proposed to sense this tension. (d) Checkpoint silencing. Several checkpoint proteins have been shown to move from kinetochores along microtubules to the spindle poles, where they are released. This pathway depends on dynein, and might also require the functions of Rod, Zw10 and CENP-E.