Ribosomes

Ribosome definition

Cell biologist George Emil Palade first observed the ribosomes inside a cell using an electron microscope in the 1950s. He described them as “small particulate components of the cytoplasm”. A few years later, the term ribosome was proposed during a symposium and was later widely accepted by the scientific community. The word comes from “ribo” = ribonucleic acid (RNA), and the Latin word “soma” = body, meaning a body of ribonucleic acid. This name refers to the composition of ribosomes, which are composed of ribosomal RNA and proteins.

The ribosome’s function in protein synthesis is so critical for all cellular activities that two Nobel prizes have been awarded to research teams that study the ribosome.

Ribosome structure

Ribosomes comprise two subunits (Fig. 1), one large and one small, with both subunits made up of ribosomal RNA (rRNA) and proteins. These rRNA molecules are synthesized by the nucleolus inside the nucleus and combined with proteins. The assembled subunits exit the nucleus to the cytoplasm. Under a microscope, ribosomes look like small dots that can be found free in the cytoplasm, as well as bound to the continuous membrane of the outer nuclear envelope and the endoplasmic reticulum (Fig. 2).

How do ribosomes know how to synthesize a specific protein? Remember that the nucleus previously transcribed the information from genes into messenger RNA molecules -mRNA- (the first step in gene expression). These molecules ended up exiting the nucleus and are now in the cytoplasm, where we also find the ribosomes. In a ribosome, the large subunit is located on top of the small one, and in the space in between the two, the mRNA sequence passes through to be decoded.

The ribosome small subunit “reads” the mRNA sequence, and the large subunit synthesizes the corresponding polypeptide chain by linking amino acids. This corresponds to the second step in gene expression, the translation from mRNA to protein. The amino acids needed for polypeptide synthesis are brought from the cytosol to the ribosome by another type of RNA molecule, appropriately called transfer RNA (tRNA).

Ribosomes that are free in the cytosol or bound to a membrane have the same structure and can interchange their location. Proteins produced by free ribosomes are usually used within the cytosol (like enzymes for sugar breakdown) or are destined for mitochondria and chloroplasts membranes or imported to the nucleus. Bound ribosomes generally synthesize proteins that will be incorporated into a membrane (of the endomembrane system) or that will exit the cell as secretory proteins.

Protein synthesis is essential for cell activity, they function as diverse vital molecules, including enzymes, hormones, antibodies, pigments, structural components, and surface receptors. This essential function is evidenced by the fact that all cells, prokaryotic and eukaryotic, have ribosomes. Although bacterial, archaeal, and eukaryotic ribosomes differ in subunits size (prokaryotic ribosomes are smaller than eukaryotic ones) and specific rRNA sequences, they all are composed of similar rRNA sequences, have the same basic structure with two subunits where the small one decodes mRNA, and the large one joins amino acids together. Thus, it seems that ribosomes evolved early in the history of life, which also reflects the common ancestry of all organisms.