Key Concepts of Section 10.5

• A large precursor pre-rRNA (13.7 kb in humans) transcribed by RNA polymerase I undergoes cleavage, exonucleolytic digestion, and base modifications to yield mature 28S, 18S, and 5.8S rRNAs, which associate with ribosomal proteins into ribosomal subunits.

• Transcription and processing of pre-rRNA occur in the nucleolus. The 5S rRNA component of the large ribosomal subunit is synthesized in the nucleoplasm by RNA polymerase III.

• Approximately 150 snoRNAs, associated with proteins in snoRNPs, base-pair with specific sites in pre-rRNA, where they direct ribose methylation, modification of uridine to pseudouridine, and cleavage at specific sites during rRNA processing in the nucleolus.

• Group I and group II self-splicing introns, and probably snRNAs in spliceosomes, all function as ribozymes, or catalytically active RNA sequences, that carry out splicing by analogous transesterification reactions requiring bound Mg²⁺ ions (see Figure 10-44).

• Pre-tRNAs synthesized by RNA polymerase III in the nucleoplasm are processed by removal of the 5′-end sequence, addition of CCA to the 3′ end, and modification of multiple internal bases (see Figure 10-45).

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  Some pre-tRNAs contain a short intron that is removed by a protein-catalyzed mechanism distinct from the splicing mechanisms used by pre-mRNAs and self-splicing introns.

• All species of RNA molecules are associated with proteins in various types of ribonucleoprotein particles, both in the nucleus and after export to the cytoplasm.

• Nuclear bodies are functionally specialized regions in the nucleus where interacting proteins form self-organized structures. Many of these bodies, including the nucleolus, are regions of assembly of RNP complexes.