The nucleolus

In almost all living cells of eukaryotic organisms in the nucleus can be seen one or more usually rounded Taurus the size of 1-5 microns, strongly refracting the light is the nucleolus, or nucleoli (nucleolus). The General properties of the nucleolus is the ability to turn various dyes, especially key. This basophilia is determined by the fact that nuclei are rich in RNA. The nucleolus is the dense structure of the nucleus is derived chromosome, one of the loci with the highest concentration and activity of RNA synthesis in the interphase. It is not an independent structure or organelle.

It is now known that the nucleolus is the site of ribosomal RNA (rRNA) and the ribosome, which is the synthesis of polypeptide chains in the cytoplasm.

The formation of nuclei and the number associated with the activity and the number of segments of chromosome - nucleolus organizers, which are located mostly in areas of secondary constrictions; the number of nucleoli in cells of this type can change due to the merger of nucleoli or by changing the number of chromosomes with nucleolar organizers. In the study of fixed cells around the nucleolus always shows the area of condensed chromatin, often identified with the chromatin nucleolar organizer. This Okladnikova the chromatin, according to electron microscopy, is an integral part of the complex structure of the nucleolus. DNA nucleolar organizer presents multiple (several hundred) copies of rRNA genes: each gene is synthesized macromolecular precursor RNA, which turns into more short RNA molecules that comprise subunits of the ribosome multiple (several hundred) copies of rRNA genes, at each of these genes, synthesized high molecular weight precursor RNA, which is converted into a short RNA molecules belonging to the ribosomal subunits.

Scheme participation of nucleoli in the synthesis of cytoplasmic proteins can be represented as follows: DNA nucleolar organizer formed precursor rRNA, which is in the area of nucleolar protein dresses, here are assembled ribonucleoprotein particles - subunits of ribosomes; subunit departing from the endosome to the cytoplasm and participate in the process of protein synthesis.

The nucleolus is inhomogeneous in their structure: the light microscope can see it fine-fiber organization. In the electron microscope revealed two main components: granular and fibrillar. The diameter of the granules is about 15-20 nm thick fibrils - 8.6 nm.

Fibril component may be concentrated in a central part of the nucleolus and granular - peripherally. Often granular component forming filamentous structures - nukleolonemy thickness of about 0.2 microns. Fibrillar component of the nucleolus is a ribonucleoprotein strands predecessors ribosomes, and granules - maturing ribosomal subunit. In the area of the fibrils can identify DNA regions of nucleolar organizers.

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