Nucleolus

Nucleolus (plural nucleoli) is the largest and most noticeable structure within the nucleus of a eukaryotic cell. They are composed up of DNA, RNA and proteins. They vary in number from one to many nucleoli in the nucleus. They are complex, not membrane enclosed organelle of a eukaryotic cell that separates granular and fibrillar regions.  They are present in nondividing cells but generally disappears during the beginning of mitosis and then begins to reassemble in telophase around nucleolar organizer.

They are the site of ribosome biogenesis and forms rRNA (ribosomal RNA). The rRNA combines with ribosomal proteins and forms ribosomal subunits. These ribosomal subunits are immature and they leave the nucleus through nuclear envelope pores. Then, they matures in the cytoplasm. Nucleoli also help in the assembly of signal recognition particles, modification of tRNA (transfer RNA) and also play a major role in cell’s response to stress.