A. rRNA and Ribosomes
The synthesis and processing of rRNAs are coincident with the assembly of the ribosomal subunits, as shown below.
The 45S pre-rRNAs initially bind to ribosomal proteins in the nucleolus (that big nuclear body!) to initiate assembly and then and serve as a scaffold for the continued addition of ribosomal proteins to both the small and large ribosomal subunits. After the 5S rRNA added to the nascent large ribosomal subunit, processing (cleavage) of 45S rRNA is completed and the subunits are separated. The separated ribosomal subunits exit the nucleus o the cytoplasm where they will associate with mRNAs to translate new proteins. To better understand what is going on, try summarizing what you see here in the correct order of steps. You can also see this process animated at this link: here.
The 5’ methyl guanosine cap and the poly(A) tail collaborate to facilitate exit of mRNAs from the nucleus into the cytoplasm. We now understand that proteins in the nucleus participate in the export process. A nuclear transport receptor binds along the mature (or maturing) mRNA, a poly-A-binding protein binds along the poly-A tail of the message, and another protein binds at or near the methyl guanosine CAP itself. These interactions enable transport of the mRNA through nuclear pores. After the mRNA is in the cytoplasm, the nuclear transport receptor re-cycles back into the nucleus while a translation initiation factor replaces the protein bound to the CAP. The nuclear transport process is summarized in the illustration below.
See a more detailed description of mRNA transport from the nucleus at this link: here. The mature mRNA, now in the cytoplasm, is ready for translation. Translation is the process of protein synthesis mediated by ribosomes and a host of translation factors (including the initiation factor in the illustration above. The genetic code directs polypeptide synthesis during translation. Details of translation will be discussed shortly.