repeated rRNA genes are very actively transcribed and processed. Therefore in electron micrographs, the Each of the tandemly arrayed repeated rrNa transcription units is surrounded by 100 or so densely packed growing rRNA fibrils or cleaved intermediate rRNA precursor fibrils forming a structure that looks like a Christmas tree: the dark granule at the base of each fibril is Rna polymerase I, and the characteristic knob at the 5'end of pre-rrNa fibril is the attached U3 SnRNP required for processing cleavage. The nontranscribed spacer DNA regions between transcription units are devoid of nascent rna fibrils In addition to cleavage, the processing of pre-rRNA involves in the modifying of the pre-rRNA resulting both from the addition of methyl groups to specific bases and ribose residues and from the conversion of uridine to pseudouridine
repeated rRNA genes are very actively transcribed and processed. Therefore in electron micrographs, the Each of the tandemly arrayed repeated rRNA transcription units is surrounded by 100 or so densely packed growing rRNA fibrils or cleaved intermediate rRNA precursor fibrils, forming a structure that looks like a Christmas tree: the dark granule at the base of each fibril is RNA polymerase I, and the characteristic knob at the 5’end of pre-rRNA fibril is the attached U3 snRNP required for processing cleavage. The nontranscribed spacer DNA regions between transcription units are devoid of nascent RNA fibrils. In addition to cleavage, the processing of pre-rRNA involves in the modifying of the pre-rRNA resulting both from the addition of methyl groups to specific bases and ribose residues and from the conversion of uridine to pseudouridine
Ribosome assembly The formation of ribosomes involves the assembly of the ribosomal precursor rna with both ribosome proteins and 5s rRNA Ribosome proteins are imported from the cytosol into the nucleolus and begin to assemble on the pre-rrNa prior to its cleavage. As the pre-rRNA is processed remaining ribosome proteins and 5s rrNa( which is synthesized outside of nucleolus by rna polymerase IIi) are then incorporated into preribosomal particles. Final preribosomal particles are exported to the cytoplasm, forming the active 40S and 60S ribosome subunits. Most of the preribosomal particles in the nucleoli represent the precursor of the larger subunit because the smaller ribosome subunit mature more rapidly then the larger subunit As above mentioned, during the formation of ribosomes there is a considerable amount of molecular traffic in one way or shuttling rapidly of carrier-snoRNPs between the nucleus and the cytosol
Ribosome assembly The formation of ribosomes involves the assembly of the ribosomal precursor RNA with both ribosome proteins and 5S rRNA. Ribosome proteins are imported from the cytosol into the nucleolus and begin to assemble on the pre-rRNA prior to its cleavage. As the pre-rRNA is processed, remaining ribosome proteins and 5S rRNA ( which is synthesized outside of nucleolus by RNA polymerase III) are then incorporated into preribosomal particles. Final preribosomal particles are exported to the cytoplasm, forming the active 40S and 60S ribosome subunits. Most of the preribosomal particles in the nucleoli represent the precursor of the larger subunit, because the smaller ribosome subunit mature more rapidly then the larger subunit. As above mentioned, during the formation of ribosomes there is a considerable amount of molecular traffic in one way or shuttling rapidly of carrier-snoRNPs between the nucleus and the cytosol