A cell is the basic unit of living organisms that is capable of reproducing itself exactly. That is, although eukaryotic cells (such as plant cells and animal cells) contain components, which are called organelles, those components do not reproduce themselves independently of the cell of which they are a part.
The nucleolus is a dense spherical structure within the nucleus of a cell. It contains ribonucleic acid (RNA) for the synthesis of ribosomes and also has an important role in the production of proteins and RNA.
The nucleolus is a part of the nucleus of the cell that disappears during cell division.
See also the functions of cell organelles.