Figure 257

Electron micrograph of the nucleolus. This nucleolus from a nerve cell shows the fibrillar (F) and granular (C) materials. Such a network of both materials is referred to as the nucleolonema. The rRNA, DNA-con-taining genes for the rRNA, and specific proteins are localized in the interstices of the nucleolonema. x 15,000.

bosomal subunits is localized in the interstices of this network.

rRNA genes are transcribed by RNA polymerase I, and subunits of rRNA are assembled using ribosomal proteins imported from the cytoplasm. The partially assembled ribosomal subunits leave the nucleus via nuclear pores (see below), to be fully assembled into ribosomes in the cytoplasm.

The nucleolus stains intensely with hematoxylin and basic dyes and metachromatically with thionine dyes

That the basophilia and metachromasia of the nucleolus are due to the phosphate groups of the nucleolar RNA is confirmed by predigestion of specimens with ribonuclease (RNAse), which abolishes the staining. DNA is present in the nucleolus but in such small amounts that it appears Feulgen negative when examined in the light microscope. However, Feulgen-positive material, nucleolus-associated chromatin, often rims the nucleolus.

inner and outer membrane of nuclear envelope nuclear pore nuclear pore

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