The Animal Cell Nucleolus: Function and Structure

The animal cell nucleolus is a structure within the nucleus that produces ribosomes, the cell’s protein-synthesis machinery. It is a dense, non-membrane-bound organelle that plays a part in a cell’s response to stress and the formation of signal recognition particles. The nucleolus is composed of proteins, DNA, and RNA. The number and appearance of nucleoli can vary depending on the cell’s metabolic state; for instance, an active cell involved in wound healing will have a prominent nucleolus, while an inactive one will have a small one.

The Nucleolus Within the Animal Cell

An animal cell contains the cytoplasm and a nucleus, which houses the cell’s genetic material and is its control center. Within the nucleus is the nucleolus, a spherical structure that can occupy a significant portion of the nuclear volume and is its largest component.

Unlike the nucleus, which is enclosed by the nuclear envelope, the nucleolus is a highly organized subdomain without a membrane. The number of nucleoli is fixed for each species and relates to the cell’s protein production needs. Cells with high rates of protein synthesis, such as nerve cells, have prominent nucleoli, whereas cells like sperm that do not produce proteins lack them.

Internal Structure of the Nucleolus

The nucleolus has a highly organized internal structure consisting of three main components: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). This tripartite organization is characteristic of higher eukaryotes. These dynamic sub-compartments interact to carry out the functions of the nucleolus.

The fibrillar center is the innermost region where the genes for ribosomal DNA (rDNA) are located, along with the proteins necessary for their transcription. Surrounding the FC is the dense fibrillar component, which contains the protein fibrillarin and is the site where the initial processing of ribosomal RNA (rRNA) occurs.

The outermost region is the granular component, where later stages of rRNA processing and the assembly of ribosomal proteins with rRNA take place. The GC contains the protein nucleophosmin, which is involved in ribosome biogenesis. The dense concentration of rRNA complexed into ribosomal subunits in these regions gives the nucleolus its dark-staining appearance under a microscope.

Ribosome Production Center

The primary function of the nucleolus is manufacturing the subunits that form ribosomes, the cellular machinery for synthesizing proteins. This process, ribosome biogenesis, begins with the transcription of ribosomal DNA.

The process starts in the fibrillar center, where rDNA genes are transcribed into a precursor ribosomal RNA molecule. This pre-rRNA molecule then moves into the dense fibrillar component for initial processing, where it is cleaved and modified.

In the granular component, the processed rRNA molecules are combined with ribosomal proteins that were synthesized in the cytoplasm and imported into the nucleolus. This assembly forms the small (40S) and large (60S) ribosomal subunits. Once formed, these subunits are exported to the cytoplasm, where they join to form a complete ribosome and begin protein synthesis.

Additional Nucleolar Functions

Beyond ribosome biogenesis, the nucleolus has several other functions. It participates in the cell’s response to stress by capturing and immobilizing certain proteins, a process known as nucleolar detention. This sequestration prevents these proteins from interacting with their usual partners, providing a layer of regulation.

The nucleolus is also involved in:

• Assembling other ribonucleoprotein complexes, such as the signal recognition particle (SRP).
• Processing other types of RNA molecules, including some small RNAs.
• Regulating the cell cycle and mitosis.
• Cellular aging, based on connections suggested by some research.

Nucleolar Dynamics and Cellular Impact

The formation of the nucleolus is tied to specific chromosomal regions known as nucleolar organizing regions (NORs). These regions contain the tandemly repeated genes for ribosomal RNA. At the end of cell division, the nucleolus assembles around these NORs, recruiting the machinery for ribosome production.

The nucleolus is a dynamic structure that changes during the cell cycle. As a cell prepares to divide, the nucleolus disassembles. Following chromosome separation into two daughter cells, the nucleolus reassembles in each new nucleus.

Nucleolar dysfunction is associated with human diseases termed “nucleolopathies.” Alterations in the size, number, or activity of nucleoli are observed in cancer cells, reflecting their increased demand for protein synthesis to support rapid growth. This makes the nucleolus a potential target for new cancer therapies.