The Saccharomyces nucleus is an ovoid structure localized off to one side of the vacuole in the unbudded cell. It contains the chromosomes and in the electron microscope one can observe a nucleolus. For an in-depth review of nuclear structure and function the reader is referred to Wente et al. (1997). The Saccharomyces nuclear envelope that separates the contents of the nucleus from the cytoplasm consists of a double membrane, each having a lipid bilayer. One unique feature of Saccharomyces is the fact that the nuclear envelope remains intact throughout all stages of cell division and mating. This necessitates several adaptations to ensure proper chromosome segregation and karyokinesis. One of these is the spindle pole body described below.
Numerous nuclear pore complexes span the double membranes of the nuclear envelope and create channels through which proteins, RNAs, and even larger complexes like ribosomal subunits can pass in both directions, often with the aid of specialized import and export protein chaperons. In Saccharomyces, the outer nuclear membrane faces the cytoplasm and is continuous with the endoplasmic reticulum (see below). Much of the outer membrane is covered with bound ribo-somes and is in essence functioning as the rough ER.
In addition to the nuclear pore complexes, the spindle pole body (SPB) spans the nuclear envelope (see Figure 3.8). It is a complex structure consisting of three plaques, platelike structures, bound together by cross-bridging proteins (reviewed in Botstein et al., 1997). The middle plaque is embedded in and spans both membranes of the nuclear envelope. Microtubules (see below) are attached to the plaques that lie on the cytoplasmic and nuclear sides of this central plaque. It is believed that the SPB serves as a microtubule-organizing center and allows for the polymerization of tubulin in a polarized fashion. During cell division it also functions as a centrosome-like element (to be discussed below) and as the initiating site of spore wall formation in meiosis.
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