Which structure is called as little nucleus? What is the role of this structure?

The nucleolus (plural: nucleoli) is often called the "little nucleus" because it appears as a dense, spherical structure within the nucleus. Despite its small size, the nucleolus is not surrounded by its own membrane and is actually a specialized region within the nucleus where specific genetic activity occurs. It is clearly visible under a microscope as a darker spot inside the nucleus of most eukaryotic cells.

The primary role of the nucleolus is the synthesis and assembly of ribosomes, the cell's protein-making machinery. It contains specific regions of DNA with genes that code for ribosomal RNA (rRNA), which is transcribed and then combined with ribosomal proteins imported from the cytoplasm. These components are assembled into the large and small subunits of ribosomes within the nucleolus before being exported to the cytoplasm where they function in protein synthesis. Cells that produce large amounts of protein, such as pancreatic cells that make digestive enzymes, typically have larger and more prominent nucleoli to meet the high demand for ribosomes. The nucleolus essentially serves as the ribosome factory of the cell, making it indispensable for cellular growth and protein production.