11. 4. The nucleolus The most prominent substructure within the nucleus during interphase is the nucleolus. 100 years ago, one found a big spherical structure, called nucleolus within the nucleus, but up until 1960s, it was not identified that the nucleolus is a ribosome production factory, designed to fulfill the need for large scale transcription and processing of rrna and assembly of ribosomal subunits The nucleolus is organized around the chromosoma Fo 1.r RNa genes and transcription and processing ofrRN regions that contain the genes for the 5.8S, 18S and 28S IRNa within a single transcription unit for a 45S pre-pRNA which is transcribed within the nucleolus by rna polymerase 1. The 5S RNA, another component of 60S large) ribosomal subunit is transcribed outside of the nucleolus by rNa polymerase Il.11.4. The nucleolus The most prominent substructure within the nucleus during interphase is the nucleolus. 100 years ago, one found a big spherical structure,called nucleolus within the nucleus, but up until 1960s, it was not identified that the nucleolus is a ribosome production factory, designed to fulfill the need for large scale transcription and processing of rRNA and assembly of ribosomal subunits. 1.r RNA genes and transcription and processing of rRNAs The nucleolus is organized around the chromosomal regions that contain the genes for the 5.8S, 18S and 28S rRNA within a single transcription unit for a 45S pre-rRNA, which is transcribed within the nucleolus by RNA polymerase I. The 5S RNA, another component of 60S (large) ribosomal subunit is transcribed outside of the nucleolus by RNA polymerase III