CBSE Class 9 Answered

The nucleolus is a non-membrane bound structure composed of protein and nucleic acids found within the nucleus.

Each diploid cell in the human body features only one nucleolus, though immediately after cell division ten tiny nucleoli appear before they coalesce into a single, large nucleolus. This is because nucleoli are formed at certain chromosome sites usually referred to as nucleolus organizer regions (NORs), and two copies of five different human chromosomes contain NORs. A NOR is composed of tandem repeats of rRNA genes. The DNA found at chromosomal NORs encodes the genes for ribosomal RNA (rRNA). At the onset of mitosis, the single nucleolus present in a human cell disappears, and subsequent to the process, the formation of the new nucleolus, which is created from the ten smaller nucleolus-like structures that develop from the NORs, can be observed.

The nucleolus is comprised of granular and fibrillar components, as well as an ill-defined matrix, in addition to DNA. The granular material consists of ribosomal subunits that have already been formed but have not yet matured and are waiting to be exported to the cytoplasm. The threadlike fibrillar part of a nucleolus is predominantly composed of rRNA molecules and associated proteins that have joined together to form fibrils. It has not yet been determined exactly how the various components of the nucleolus are secured together and organized.

Proteins present in nucleolus are called Nucleolar proteins.

(In eukaryotes, the protein–RNA complexes localize to the nucleolus and are called small nucleolar RNPs (snoRNPs)