In eukaryotic cells, the primary function of the nucleolus is to assemble ribosomes. In a cell nucleus, this is the biggest component.
The Nucleolus Before better understanding the function of the nucleolus, it is important to know what it is. Inside the borders of the nucleus, the nucleolus is the largest organelle. It is not a static structure. In the initial G1 phase, this structure will disassemble and put itself back together during mitosis.
It is only in eukaryotic cells that there is a structural nucleolus. The structural components of this include chains of DNA and RNA. There are both fibrillary and granular components that make it up. In plant cells, there is only a nucleolar vacuole.
Forming the Subunits for a Ribosome This is the primary function of the nucleolus. It works to create and assemble the subunits that are responsible for forming the ribosome. Protein synthesis occurs in the ribosome. By assembling these subunits, it also plays a small role in protein synthesis. In cells, half of the total RNA production is carried out by the nucleolus. There are several steps associated with ribosomal subunit assembly and all must occur exactly as they are supposed to in order to produce effective subunits.
Nucleolar Sequestration This function of the nucleolus results in protein immobilization. This means that with their pairing partners, the proteins are not able to interact. The nucleolus can sequester a number of proteins, including Mdm2, pVHL and hTERT.
The Mdm2 protein is one that is present in humans. It works to negatively regulate the p53 tumor suppressor. This nuclear protein is important for regulating transcription. The pVHL protein is a type of tumor suppressor that is encoded by the VHL gene in humans. When the VHL gene is mutated, it is associated with von Hippel-Lindau disease. The hTERT protein is part of the telomerase complex's most important unit. It works with the telomerase RNA component and it is one of the enzyme telomerase subunits.
mRNA Biogenesis mRNA biogenesis is a task taken on by multiple structures, including the nucleolus. This refers to the production of new messenger RNA molecules. These play an important role in taking genetic information from DNA and giving it to the ribosome. The nucleolus also aids in ribonucleoprotein assembly and RNA metabolism.
Sensing Cellular Stress When stress occurs, cells usually are able to quickly respond and they compensate by changing their metabolism. Stress signaling in cells usually results in programmed cell death or an arrest of the cell cycle. This will ultimately depend on how significant the stress is and an individual cell's ability to recover.
The nucleolus is considered to be a central hub for aiding a cell in responding to and coping with stress. Under stress conditions, the nucleolus protein content tends to change. Proteomic studies have been done to see how the nucleolus responds to a variety of stressful stimuli to get this information. Such stimuli included DNA damage and viral infection. It further showed the nucleolus undergoing complex reorganization as part of the stress response.