Cell nucleus
Within the nucleus is a structure called a nucleolus. It is made at a nucleolus organizer region (NOR). This is a chromosomal region around which the nucleolus forms. Inside the nucleolus ribosomes are made. These are exported through the nuclear pore complexes to the cytoplasm. There they  work to build proteins. They become attached to the endoplasmic reticulum if they are making membrane proteins.