Spindle pole body

The spindle pole body (SPB) is the microtubule organizing center in yeast cells, functionally equivalent to the centrosome. Unlike the centrosome the SPB does not contain centrioles. The SPB organises the microtubule cytoskeleton which plays many roles in the cell. It is important for organising the spindle and thus in cell division.

SPB structure in Saccharomyces cerevisiae

The molecular mass of a diploid SPB, including microtubules and microtubule associated proteins, is estimated to be 1–1.5 GDa whereas a core SPB is 0.3–0.5 GDa. The SPB is cylindrical multilayer organelle. These layers are: an outer plaque (OP), which connects to the cytoplasmic microtubules (cMT); a first intermediate layer (IL1) and an electrondense second intermediate layer (IL2); an electrondense central plaque (CP), which is at the level of the nuclear envelope and is connected to it by hook-like structures, an ill-defined inner plaque (IP); and a layer of the inner plaque that contains capped nuclear microtubules (nMT) ends. The central plaque and IL2 appeared as distinct but highly ordered layers. The other layers (MT ends, IP, IL1, and OP) do not show ordered packing. The location of the inner and outer plaques in relation to the nuclear membranes is maintained during the entire cell cycle. One side of the central plaque is associated with an electron-dense region of the nuclear envelope termed the half bridge. The SPB has constant height size (the inner plaque to outer plaque distance) for about 150 nm, but its diameter changes during cell cycle, e.g. in haploid cells, the SPB grows in diameter from 80 nm in G1 to 110 nm in mitosis. The SPB diameter depends on DNA content. A larger SPB diameter increases microtubule nucleation capacity of the SPB, which is important for chromosome segregation.

Spindle apparatus

In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a process that produces genetically identical daughter cells, or the meiotic spindle during meiosis, a process that produces gametes with half the number of chromosomes of the parent cell.

Besides chromosomes, the spindle apparatus is composed of hundreds of proteins.Microtubules comprise the most abundant components of the machinery.

Spindle structure

Attachment of microtubules to chromosomes is mediated by kinetochores, which actively monitor spindle formation and prevent premature anaphase onset. Microtubule polymerization and depolymerization dynamics drive chromosome congression. Depolymerization of microtubules generates tension at kinetochores; bipolar attachment of sister kinetochores to microtubules emanating from opposite cell poles couples opposing tension forces, aligning chromosomes at the cell equator and poising them for segregation to daughter cells. Once every chromosome is bi-oriented, anaphase commences and cohesin, which couples sister chromatids, is severed, permitting the transit of the sister chromatids to opposite poles.