Though a nucleole is tiny, it's the largest part of a eukaryotic cell's nucleus. A nucleole is made up of protein, RNA, and DNA.
In biology, nucleole is another word for nucleolus; both words mean "small nucleus," from a Latin word meaning "a little nut." The microscopic nucleole is often described by scientists as a "ribosome factory," churning out the tiny particles that produce important proteins from amino acids. All of this work happens in eukaryotes, cells with membrane-enclosed nuclei.