The ribosome is a ribonucleoprotein complex allowing the translation of genetic information encoded by messenger RNAs into proteins.

Ribosomes are ribonucleoprotein complexes found in the cytosol of eukaryotic and prokaryotic cells. Ribosomes convert the genetic information encoded by messenger RNAs (mRNAs) into chains of amino acids that make up proteins.

The ribosome arises within the cell by biogenèse, a very complex process that requires a large number of maturation steps.

The number of ribosomes within a cell varies depending on the nature of the cell: it increases in protein-making cells such as hepatocytes. For example, there are 10 ribosomes in liver cells (hepatocytes).

The ribosome is made up of water, ribosomal RNA, and protein.

In prokaryotes the length of the ribosome is 29nm by 21nm in width while in eukaryotes the length is 32nm by 22nm in width.

The ribosome has a slightly elliptical longitudinal shape.

A division into 2 sub-units

A transverse groove divides the ribosome into two ribosomal subunits:

• la large ribosomal subunit which contains 33 L (Long) proteins;
• la small ribosomal subunit which contains 21 S (Short) proteins.

Each of these subunits is characterized by its sedimentation coefficient expressed in Svedberg units. Thus the sedimentation coefficient is:

• 80S for the entire ribosome of eukaryotes and 70S for that of prokaryotes;
• 60S for the large subunit of eukaryotes and 50S for that of prokaryotes;
• 40S for the small subunit of eukaryotes and 30S for that of prokaryotes.

4 linking sites

The ribosome has four binding sites for RNA:

• a binding site for messenger RNA (mRNA);
• an amino-acyl-tRNA binding site or site A (this site binds the transfer RNA or incoming tRNA molecule carrying an amino acid);
• a peptidyl-tRNA site or P site, (this site fixes the transfer RNA or tRNA molecule linked to the growing end of the polypeptide chain);
• an amino-acyl-tRNA exit site or E site.

Free ribosomes bound to membranes

Within the cell, ribosomes can be:

• free within the cytosol in the inactive form or grouped in active polyribosomes;
• linked by their large subunit to the membrane of the endoplasmic reticulum or to the outer membrane of the nuclear envelope.

The main function of ribosomes is the proteosynthesis that is to say the set of biochemical reactions which use amino acids for the synthesis of proteins.

DNA carries the information necessary for the establishment of an amino acid in the correct position in a polypeptide chain. The information in DNA is transcribed in the cell nucleus into messenger RNA (mRNA).

A messenger RNA (mRNA) molecule transfers this information to the ribosome, which is located in the cytosol where protein synthesis will take place.

The main role of the ribosome is to read the message from DNA (via messenger RNA or mRNA) and translate it into protein. This translation takes place using machinery capable of reading the messenger RNA (mRNA) molecule and associating amino acids with each other. This machinery requires the presence of transfer RNA (tRNA) molecules which transport amino acids to the ribosome.

Proteosynthesis takes place in 3 stages respectively called:

• initiation;
• elongation;
• termination.

It was in the 1950s that the scientist of Romanian origin George Palade and his collaborator, the biologist Philip Siekevitz, observed by transmission electron microscopy, “molecules” attached to the endoplasmic reticulum. This is the first description of what will later be called the ribosome.

In 1974, George Palade received the Nobel Prize in physiology and medicine for this discovery (at the same time as the Belgian scientists Albert Claude and Christian de Duve for the discovery of the endoplasmic reticulum).

Then, for the second time in 2009, a Nobel Prize in chemistry was awarded to professors Ada Yonath, Thomas Steitz and Venkatraman Ramakrishnan for their work on the structure and mode of action of the ribosome.