92 Chapter 3 • The Chromosomal Basis of Inheritance 3. Anaphase: The sister chromatids are pulled to opposite ends of the cell by microtubules that attach to the centromeres. The microtubules are part of the nuclear spindle, a set of parallel ﬁbers running from one pole of the cell to the other. Nuclear spindle ﬁbers provide the motive force that pulls apart the chromosomes or chromatids in mitosis and meiosis (Figure 3-25). In nuclear division, spindle ﬁbers form parallel to the cell axis, connected to one of the cell poles. These spindle ﬁbers are polymers of a protein called tubulin . Each centromere acts as a site to which a multiprotein complex called the kinetochore binds (Figure 3-26). The kinetochore acts as the site for attachment to spindle ﬁber microtubules. From one to many microtubules from one pole attach to one kinetochore, and a similar number from the opposite pole attach to the kinetochore on the homologous chromatid. Although the microtubules of the spindle appear like ropes,
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