Structural Biochemistry/Cell Organelles/Nucleolus

Overview
The nucleolus, or plural nucleoli, is normally a circular structure composed of proteins and nucleic acids. Nucleoli are not typical organelles for the reason that they have no lipid membrane, making it with of the few non-membrane bound organelles in the cell. The nucleolus is located within the nucleus of eukaryote cells and is in charge of producing ribosomal RNA and the arrangement of ribosomes. The structure of the nucleoli can be seen using electron microscopy and fluorescent protein tagging can be used to view the dynamics of the nucleoli.

Structure
The nucleolus has three components:
 * 1) Fibrillar Centers (FC): FC is the place where ribosomal proteins are made.
 * 2) Dense Fibrillar Components (DFC): It has new transcribed RNA which binds to ribosomal proteins to form rRNA
 * 3) Granular Components (GC): Before ribosomes are formed, GC has rRNA that binds to ribosomal proteins.

Function
The nucleolus is the nuclear subdomain that assembles ribosomal subunits in eukaryotic cells. The nucleolar organiser regions of chromosomes, which contain the genes for pre-ribosomal ribonucleic acid (rRNA), serve as the foundation for nucleolar structure. The nucleolus disassembles at the beginning of mitosis, its components disperse in various parts of the cell and reassembly occurs during telophase and early G1 phase. Ribosome assembly begins with transcription of pre-rRNA. During transcription ribosomal and nonribosomal proteins attach to the RNA. Subsequently, there is modification and cleavage of pre-rRNA and incorporation of more ribosomal proteins and 5S rRNA into maturing pre-ribosomal complexes. The nucleolus also contains proteins and RNAs that are not related to ribosome assembly and a number of new functions for the nucleolus have been identified. These include assembly of signal recognition particles, sensing cellular stress and transport of human immunodeficiency virus 1 (HIV-1) messenger RNA.

Reference
http://en.wikipedia.org/wiki/Nucleolus