The nucleolus toccupies 25% of the nucleus and it is made up of proteins and ribonucleic acids (RNA). Its main function is to make ribosomal subunits from proteins and ribosomal RNA, also known as rRNA. This leads to the formation of incomplete ribosomes. It then sends the subunits out to the rest of the cell and forms complete ribosomes. The nucleolus plays a vital role in the making of proteins in the cell.