The nucleolus is a prominent structure within the nucleus of eukaryotic cells. It's responsible for:

* Ribosomal RNA (rRNA) synthesis: The nucleolus contains the genes that code for rRNA, and it's where rRNA is transcribed and processed.

* Ribosome assembly: rRNA molecules combine with proteins to form ribosomes, which are the protein synthesis machinery of the cell.

Since protein synthesis is essential for all living cells, both plant and animal cells need ribosomes and therefore have nucleoli.