The nucleolus is the site of rRNA synthesis and initial ribosomal assembly

The nucleolus is a nonmembranous, intranuclear structure formed by fibrillar material (pars fibrosa) and granular material (pars granulosa) (Fig. 2.57). It varies in size but is particularly well developed in cells active in protein synthesis. Some cells contain more than one nucleolus. The nucleolus consists largely of DNA loops of different chromosomes containing genes for rRNA clustered together, large amounts of rRNA, and proteins.

rRNA is present in both granular and fibrillar material and is organized, respectively, as both granules and extremely fine filaments packed tightly together. The network formed by the granular and the fibrillar materials is called the nucleolonema. DNA containing genes for the ri-

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