The ribosomes , ribonucleoproteic molecules, are present in the cells Eucaryotes and Procaryotes. Their function is to synthesize proteins by decoding the contained information in ARN messenger. Ribosomes consist of ribosomal ARN S, which carry the catalytic activity, and of ribosomal proteins. Ribosomes consist of two sub-units, smaller which “reads” the ARN messenger and larger which takes care of the synthesis of corresponding protein.

Localization

They are in the Cytoplasme, free, or associated, either with the membranes of the endoplasmic Réticulum, or with the nuclear Enveloppe, or even in certain bacteria with their internal membrane (for example at Escherichia coli). A ribosome chain is called Polysome or polyribosome. One finds also ribosomes in the Mitochondrie S and some Plaste S. ribosomes of the mitochondries and the plastids are different from cytoplasmic ribosomes.

Structure

Comprising ARN ribosomal known as ARN (or ARNr) and ribosomal proteins, they are composed of two sub-units: large (L for broad ) and a small (S for small ) sub-unit. The active site of the ribosome which catalyzes the peptide Liaison consists of ARN. The Biogenèse of ribosomes takes place in the Nucléole, a structure of the core.

• Large sub-unit: In the cytoplasmic ribosome of the Eucaryote S, it consists of three molecules of ARNr (5 S, 28 S and 5.8 S, comprising a hundred and twenty respectively, four thousand seven hundred and a hundred and sixty Nucléotide S), and of forty-nine ribosomal proteins. This large sub-unit has a molecular mass of 2,8.10⁶ Daltons, a coefficient of sedimentation of 60 S. At the Procaryotic S, this large sub-unit is characterized by a coefficient of sedimentation of 50 S; it is composed of ARN 23 S and thirty five proteins.

• Small sub-unit: In the cytoplasmic ribosome of the Eucaryote S, it is made up only of one molecule of ARNr (18 S, comprising one thousand and nine hundred nucleotides) and of thirty-three ribosomal proteins. This small sub-unit has a molecular mass of 1,4.10⁶ Daltons, a coefficient of sedimentation of 40 S. At the Procaryotic S, it consists of ARNr 16 S and of twenty-and-a proteins. Its coefficient of sedimentation is of 30 S.

On the whole, functional ribosome (compound of the two joined together sub-units) has a molecular mass of 4,2.10⁶ Daltons, a coefficient of sedimentation of 80 S at the eucaryotes and 70 S at the procaryotes.

Ribosomes 70 S of the procaryotes are sensitive to unquestionable antibiotic, contrary to ribosomes 80 S of the eucaryotes.

The mitochondries and the chloroplasts (of the vegetable cells) also contain ribosomes 70 S, which in addition to the fact that they have their own DNA and their own mechanism of reproduction, would support the thesis according to which these organoids would be in fact of the procaryotes in Symbiose with the cell.

Functions

See also: Translation (biology)

Ribosome is the " machine" ensuring the translation of the molecule of ARNm in the synthesis of the Protéine S. the genetic code ensures the correspondence between the sequence of ARNm and the sequence of synthesized polypeptide. Ribosome uses the ARN of transfer or ARNt like “adapters” between the amino-acid ARN messenger and .

The ARN messenger passes through the small sub-unit (30S or 40S) which contains the sites of fixing of ARNt. The large sub-unit contains the catalytic part which makes the synthesis of the peptide Liaison between the Amino-acid consecutive of protein. The large sub-unit also contains a tunnel by which the proteinic chain in the course of synthesis leaves.

Ribosome is moreover one engine molecular, which advances on the ARN messenger by consuming the energy provided by the hydrolysis of GTP. Several proteins, called factors of elongation, are associated with this movement, called translocation.

Related articles

• Synthesis of the proteins

• ARNm
• ARNr
• genetic ARNt
• Code
• Protein
• Ribozyme

Simple: Ribosome

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