The nucleolus is a part of the nucleus. Strictly speaking it is not an organelle as it does not have its own membrane. It appears as a dense region and can be seen using a light microscope (of course not on the sort of slides you make in school usually).
The nucleolus is a region of high transcription, the production of RNA, and is particularly associated with the synthesis of ribosomal RNA. The nucleolus disappears during cell division as transcription cannot be performed on condensed chromosomes.


