Which nuclear structure is the main site for transcription and processing of most rRNA and for assembly of early ribosomal subunit precursors?

Core Concept

The nucleolus is a dense, non-membrane-bound nuclear region where most major rRNA transcripts are produced and processed and where they associate with ribosomal proteins to form preribosomal particles. In many eukaryotes, 5S rRNA is transcribed by RNA polymerase III outside the nucleolus and later joins ribosome assembly, so the original wording was too universal.