Nucleolus
Categories: Cell biology stubs | Organelles
In biology, the nucleolus is, strictly speaking, a "suborganelle" of the nucleus, which is an organelle. Most plant and animal cells have one or more nucleoli although some do not. No membrane separates the nucleolus from the nucleoplasm. Nucleoli are made of protein and ribonucleic acid (RNA) and contain proteins as well as ribosomal RNA. It is known that they carry out the production and maturation of ribosomes; additional functions have also been suggested.
The nucleolus consists of three distinct regions: the fibrillar centres, the dense fibrillar component and the granular component. It is a consequence of ribosomal RNA (rRNA) synthesis: nucleolar organizers, special regions known as nuclear organizing regions (NOR) on some chromosomes that contain multiple copies of the genes encoding for rRNA (which is involved in protein biosynthesis), gather themselves in the same region where they transcribe the rRNA genes. Thus it can be said the nucleolus consists basically of nucleolar organizers and the transcribed rRNA (plus associated proteins).
Following synthesis, rRNA molecules are attached to proteins, forming ribosomal subunits, which leave for the cytosol through nuclear pores. These nuclear pores are known as the Nuclear Pore Complex (NPC).
The nucleolus disappears during cell reproduction, because there is no need for ribosomes.
cs:Jadérkode:Nucleolus eo:Nukleolo fr:Nucléole lt:Branduolėlis nl:Nucleolus ja:核小体 pl:Jąderko vi:Nhân tế bào