SPB Duplication Pathway in Saccharomyces Cerevisiae
Duplication of the SPB once, and only once, during each cell cycle is essential for formation of a bipolar mitotic spindle and accurate chromosome segregation. SPB duplication in S. cerevisiae can be divided into several steps. The first step occurs early in G1, when satellite material forms on cytoplasmic tip of half-bridge. During the second step half-bridge elongates and completes its nuclear and cytoplasmic faces fusion. In the same time satellite forms duplication plaque, a layered structure that is similar to the cytoplasmic half of a mature SPB. The last step of SPB duplication is insertion of the duplication plaque into the nuclear envelope and assembly of nuclear SPB components. At the end of G1 yeast cells contain two duplicated side-by-side SPBs connected by a complete bridge. Then bridge separates and SPB nucleates bipolar spindle. SPB continues to grow until mitosis, so protein components are able to incorporate into both SPBs throughout the cell cycle.
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