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The cell. 4. Nucleus.

NUCLEOLUS


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Nucleolus is an aggregation of DNA, enzymes, and other proteins in the nucleoplasm for synthesizing and processing ribosomal RNA, and for assembling of ribosomal subunits.

Several regions may be distinguished: fibrillar center, fibrillar component, and granular component.

The number and size of the nucleolus may change depending on the cell type and physiological situation of the cell.

Nucleolus is a nuclear compartment containing highly dense chromatin. It is visible at light microscopy. Mammal cells may contain from 0 to 5 nucleoli. The size of the nucleolus may change along with the cell activity, sometimes it becomes larger than 1 µm in diameter. Commonly, cells having an intense protein synthesis show large nucleoli. Nucleolus disappears during mitosis, allowing chromatin to be packaged into chromosomes.

Nucleolus

Motoneurons in the rhombencephalon of a lamprey. Nucleolus is observed as a dark dot in the nucleoplasm (arrows).

Synthesis and processing of ribosomal ARN (rRNA), and assembling of ribosomal subunits, occur in the nucleolus. The synthesis and assembling of a ribosomal subunit is a striking process. First, the expression of genes for ribosomal proteins occurs outside of the nucleolar region, then the mRNA crosses the nuclear envelope and is translated by free ribosomes in the cytoplasm. Then, these new ribosomal proteins enter again into the nucleus and arrive to the nucleolus, where they are associated with the rRNA to form a complete ribosomal subunit, either the small or the large subunit. Once assembled, ribosomal subunits cross again the nuclear envelope toward the cytoplasm, where they start working in translation. In this way, the nucleolus is a consequence of the presence of many molecules at the same place at the same time: ribosomal genes being transcripted to produce rRNA, proteins involved in the processing of these transcripts, ribosomal proteins of ribosomal subunits, plus proteins involved in subunit assembly. All together, it is estimated that around 690 different proteins are more o less permanently associated with the nucleolus.

Nucleolus

Fibrillar center is where the genes for rRNA are being expressed. Dense Fibrillar component is where rRNA is processed. Granular component is where ribosomal proteins and ribosomal RNA are assembled into ribosomal subunits.

Morphologically, several regions can be distinguished in the nucleolus: fibrillar center (nucleolar organizer), where the genes for rRNA are gathered and expressed, dense fibrillar component is located around the fibrillar center and where the processing of rRNA transcript takes place, and granular component, where ribosomal subunits are assembled.

Most of the proteins of the cell are synthesized from only one gene. For example, this is the case for erythrocyte hemoglobin. Proteins which are abundant are translated many times from many mRNAs, which in turn are the result of many transcriptions of the same gene. More than 1000 proteins can be synthesized from translations of one molecule of mRNA. So we have two amplification steps, transcription (many copies of mRNA from one gene) and translation (many proteins from one mRNA molecule). When the final product of the expression of a gen is not a protein but an RNA molecule, the second amplification step is missing. A single eukaryote cell may contain a huge amount of ribosomes and all of them are made up of rRNA. One copy of a gene for all the rRNAs may be insufficient. Cells solve this problem by having many copies of the same gen. Human cells contain 200 copies of the gen for rRNA (large precursor). These copies are distributed in 5 different chromosomes. All these copies are grouped in a small space (the nucleolar organiser), where they are transcripted by RNA polymerase I. There is one rRNA type that is not transcripted in the nucleolus: ribosomal 5S. There are around 20000 copies of the gen for 5S rRNA and it is transcripted by polymerase III outside of the nucleolus.

The large ribosomal transcript is cut in several pieces, resulting in 4 different rRNAs (18S rRNA, 28S rRNA, and 5.8S rRNA). They are packaged in the ribosomal subunits. The production and processing of transcripts is continuously carried out in the nucleus. The cutting of the transcript is carried out by other types or RNA, known as small nuclear RNAs. 5S rRNA, however, arrives from other region of the chromatin. There are different rRNA fragments in the two ribosomal subunits: 5.8S, 28S and 5S in the larger one, and 18S in the smaller one.


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Updated: 2016-05-13. 13:04