What is the structure of the nucleolus?

The nucleolus is the most prominent structure in a cell nucleus. It is a dense, spherical structure composed of DNA, RNA, and protein. The innermost region of the nucleolus is known as the fibrillar center. It contains DNA regions known as nucleolar organizer regions, which encode for genes for rRNA. The fibrillar center acts as a site for initial transcription of rRNA genes. The granular component is the outermost region of the nucleus. It consists of dense granules called ribonucleoprotein particles. These particles possess newly transcribed rRNA molecules, as well as ribosomal proteins. Lastly, the nucleus contains nuclear organizer centers, which are specific DNA sequences within the nucleus that contain genes for rRNA. Multiple copies of these genes are assembled within the nucleolus, and their transcription induces the synthesis of pre-rRNA molecules.