The most visible structure within the non dividing nucleus is the nucleolus. The nucleolus is a 11011-membranous bound structure. The function of the nucleolus is to synthesize ribosomes. Sometimes, there are two at more nucleoli. Their number depends on the species and the cell’s reproductive cycle. Two or three nucleoli are present in most of the cell.

‘Ihe nucleolus is roughly spherical. It appears as mass of densely stained granules under the electron microscope and fibers. Nucleoli are present at nucleolar organizers. ‘nese are specialized regions of some chromosomes. They contain multiple copies of genes for ribosome synthesis. Ribosomes are present in various stages of production. An actively growing cell can produce about 10.000 ribosomes per minute.

Nucleolus is without any membranous boundary, which separates it form the other nuclear material. Nucleolus synthesizes and stores ribosomal RNA (rRNA). Nucleolus is composed of two regions:

(a)    Peripheral granular region: It is composed of precursors of ribosomal subunits.



(b)    Central fibrillar region: It is composed of large molecular weight RNA and rRNA.

The ribosomes are assembled in the nucleolus These ribosomes are exported to cytoplasm through nuclear pores.

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