Nucleus and Nucleolus

The nucleus of the cell contains the cell's linear chromosomes and serves as the primary locus of inherited genetic material. Inner- and outer-pore-containing membranes define the nucleus and separate the chromosomes from the surrounding cytoplasm. Further partitioning occurs within the nucleus to generate the nucleolus, which functions as the ribosome-generating factory of the cell. Instead of additional membranes, fibrous protein complexes separate the nucleolus from the rest of the nucleus. In this structure, the nucleolus organizer (a specific part of a chromosome containing the genes that encode ribosomal RNA) interacts with other molecules to form immature large and small ribosomal subunits. Following processing, immature subunits exit the nucleolus and enter the nucleus. Eventually, mature ribosomal subunits and other molecules exit the nucleolus through the nuclear pores and enter the cytoplasm.

Table 1-1. Comparison of DNA Sizes

of Various Genetic


Genomic Element

Size in Base Pairs

Human chromosome

1-3 x 109

Bacterial chromosome

1-4 x 106

Mitochondrial chromosome




CAM plasmid


R388 plasmid




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