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The nucleolus is an approximately spherical region within the nucleus of eukaryotic cells. It consists of nucleolar organizers and ribosomal subunits. Nucleolar organizers are special regions on some chromosomes that contain multiple copies of the genes encoding for rRNA the ribosomes which are involved in protein biosynthesis. Following synthesis, the rRNA is attached to the ribosomal subunits in the nucleus before transport through the nuclear pores into the cytosol.