Transcription of rRNA The first step in ribosome biogenesis is the transcription of rRNA genes by RNA polymerase I. These genes are transcribed as a large precursor molecule (45S pre-rRNA in humans), which is subsequently processed into 18S, 5.8S, and 28S rRNA molecules.
Processing of rRNA The precursor rRNA undergoes several modification steps, including cleavage, methylation, and pseudouridylation. These modifications are essential for the proper folding and function of the rRNA. Small nucleolar RNAs (snoRNAs) and associated proteins play a critical role in guiding these modifications.
Assembly with Ribosomal Proteins The processed rRNAs are assembled with ribosomal proteins, which are synthesized in the cytoplasm and imported into the nucleus. This assembly forms the large (60S) and small (40S) ribosomal subunits in eukaryotes.
Export to the Cytoplasm Once the ribosomal subunits are assembled, they are exported to the cytoplasm through the nuclear pore complex. In the cytoplasm, the subunits undergo final maturation steps before becoming fully functional ribosomes capable of protein synthesis.
