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Sister chromatids
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Kinetochore MTs
Spindle pole
+
Astral MTs
-
-
+
MTOC
centrosome
Spindle MTs
+
Figure 3.2 Metaphase spindle anatomy and constituent MT populations. MTs are
arranged in an antiparallel array with their plus-ends oriented toward the center of
the spindle and their minus-ends toward the poles. MTs that connect to the chromo-
somes at their kinetochores are called kinetochore fibers or k-fibers (red) and may con-
sist of a single MT or bundles of MTs, depending on the cell type. At metaphase, the
sister chromatids of each duplicated chromosome are attached to opposite spindle
poles. Spindle MTs (gray) can be long, extending from the pole across the middle of
the spindle, or short, forming a tiled array. Astral MTs (green) emanate from the poles
or associated centrosomes with their plus-ends extending outward where they may
interact with the cell cortex.
cues for bipolarity. Even in the presence of multiple centrosomes, MTs of the
spindle frequently coalesce into a bipolar structure, indicating a high degree of
MT self-organization during spindle formation ( Godinho et al., 2009 ). The
spindle poles can be defined as the two ends of the spindle and are where
the highest density of MT minus-ends is found. In many cell types, this
focused arrangement of MT minus-ends is reinforced by the presence of a
MTOC or centrosome that nucleates MTs so that their minus-ends remain
tethered at or near the spindle pole. The organization of MTs at the spindle
pole and their regulation is a point of architectural variation among different
spindles that likely has important functional consequences for controlling
spindle size, chromosome segregation, and interactions with the cell cortex.
2.3. Microtubule populations within the spindle
Three different populations of MTs define the most basic features of meta-
phase spindle architecture ( Fig. 3.2 ). The most crucial population is termed
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