Nucleolus
In cell biology, the nucleolus (plural nucleoli) is a sub-organelle of the cell nucleus, which itself is an organelle. A main function of the nucleolus is the production and assembly of ribosome components. The nucleolus is roughly spherical, and is surrounded by a layer of condensed chromatin. No membrane separates the nucleolus from the nucleoplasm.
Nucleoli are made of protein and ribosomal DNA (rDNA) sequences of chromosomes.  The rDNA is a fundamental component since it serves as the template for transcription of the ribosomal RNA (rRNA) for inclusion in new ribosomes. Most plant and animal cells have one or more nucleoli, but some cell types do not have any. Since nucleoli carry out the production and maturation of ribosomes, large numbers of ribosomes are found inside them. In addition to ribosome biogenesis, nucleoli are believed to have other roles in cellular activity.
Nucleoli fragment during cell reproduction (they can no longer be seen in metaphase of mitosis, when the chromosome copies separate).  After the daughter cells complete their separation, the fragments of nucleoli fuse together around the nucleolus organizer regions (NORs) of the chromosomes.

# Structure
Study of the structure of the nucleolus has mainly concentrated on its RNA (Ribonucleic acid) synthesis  area. The nucleolus includes fibrillar centers (FC), dense fibrillar components (DFC), granular components (GC) and rDNA. Other components are interstices and chromatin inclusions. The nucleolus can be observed with the light microscope, and was much studied in the nineteenth century; its structure has more recently been clarified using the electron microscope.

## Fibrillar center (FC)
It is made up by a network of fine (4-5 nm thick) Fibrils. The shape of an FC is typically roughly globular, with the diameter ranging from about 50nm to 1 m. The number and size of FCs per nucleolus is variable, and changes with cellular activity and the need for ribosome production. Cells with lower cellular activity usually have fewer FC than others.

## Dense Fibrillar Component (DFC)
This component is also made up by very fine (3-5 nm) and densely packed fibrils. DFCs usually surround FCs when they are present and form a meshwork. As this is particularly true for activated states, the amount of DFC roughly reflects the nucleolar engagement in ribosome biogenesis. Sometimes this meshwork occupies large areas of the nucleolus, occasionally interspersed with small FCs.
During S phase of cell cycle, the increase in upstream binding factor (UBF) association may be due to the increase in its ability to compete with the histones for binding to the rDNA.

## Granular component (GC)
The granular component appears to consist of small granules with a diameter of about 15 nm. They typically form a mass surrounding the fibrillar complexes and embed the FCs and DFC. Thus a transition zone between DFC and GC can be observed. Although nucleolus is not membrane-bound, due to the presence of GC the border with the surrounding chromatin and nucleoplasm is usually distinct.

## Ribosomal DNA (rDNA)
rDNA is a set of tandemly-repeated genes coding for preribosomal RNA. Because these genes have the ability to initiate the formation of nucleoli during interphase, these segments of the chromosomes are called nucleolus organizer regions or NORs.  In the human genome, there are tandem repeats of the rDNA sequence on the short arms of each of the two copies of chromosomes 13, 14, 15, 21 and 22.

# Functions
The main role of the nucleolus is considered to be ribosome biogenesis, since its fundamental component rDNA codes for preribosomal RNA.
Additionally, recent research pointed out that the nucleolus is also responsible for the trafficking of various prominent small RNA species. The nucleolus helps them during their maturation process and route to their final cellular destination. Moreover, although nucleoli become invisible during cell division, more recent studies have found that they are involved in cell cycle regulation. Several of its non-traditional roles include interaction with viral components, the regulation of tumor suppressor and oncogene activities, signal recognition particle assembly, the modification of small RNA strands, the control of aging and modulating telomerase function.