| (a) Basic features of the mitotic spindle of somatic animal cells in metaphase. The microtubules (green) have one end (the minus end) embedded in the centrosome and the other (plus end) pointing outward. Numerous motors and other proteins (represented by Xs) cross-link the minus ends of microtubules at the spindle poles and the plus ends of interpolar microtubules in the spindle midzone. Additional motor and other proteins link astral microtubules to the cell cortex, kinetochore microtubules to the kinetochore, and interpolar microtubules to the chromatid arms (not shown). For simplicity, this diagram includes only two sister-chromatid pairs and a fraction of the thousands of microtubules that typically exist in the spindle. (b) Light micrograph of a centrosome-based spindle (stained red) from a salamander. (c) The mitotic spindle in plant cells is similar to that of animal cells except that there are no centrosomes at the spindle poles and no astral microtubules. (d) Light micrograph of an acentrosomal spindle from the African blood lily. (e) In budding yeast, the metaphase spindle is composed of microtubules that emanate from the nuclear face of a pair of spindle pole bodies embedded in the nuclear envelope. For simplicity, only two of the 16 sister-chromatid pairs are shown. The budding yeast spindle is composed of only about 18 intra-nuclear microtubules from each pole: one for each sister-chromatid pair and one or two interpolar microtubules. A small number of astral microtubules radiate out from the spindle poles and attach to the cell cortex to help position the nucleus in the bud neck. Panels (b) and (d) courtesy of Andrew Bajer. |
