The chief structure in the nucleus of a cell is the nucleolus. The nucleolus produces ribosomes, which move out of the nucleus to positions along the endoplasmic reticulum. The ribosomes are critical in protein synthesis.
The main role of the nucleolus is to direct the assembly of proteins within the nucleus of a cell. It is the part of the nucleus that contains most of the genes that do the coding for the synthesis of the ribosomal RNA.