Final answer:
Posttranscriptional modifications of mRNA occur in the nucleus of eukaryotic cells, including the addition of a 5' cap, splicing, and addition of a poly-A tail. Ribosomal RNA components are synthesized in the nucleolus.
Step-by-step explanation:
The three posttranscriptional modifications often seen in the maturation of mRNA in eukaryotes occur in the nucleus. These modifications include the addition of a 5' methylguanosine cap, the splicing of introns and reconnection of exons, and the addition of a poly-A tail at the 3' end. After these modifications, the mature mRNA is then exported from the nucleus to the cytoplasm for translation. Similarly, the RNA components of ribosomes are synthesized in the nucleolus, which is located within the nucleus.