They can be thought of as a factory that builds a protein from a set of genetic instructions. Ribosomes are found in the cytosol (the internal fluid of the cell) of all cells. . Ribosomes consist of two subunits (Fig. 1) that fit together (Fig. 2) and work as one to translate the mRNA into a polypeptide chain (Fig.3). Each subunit consists of one or two very large RNA molecules (known as ribosomal RNA or rRNA) and several smaller protein molecules.
Figure 1 : The subunits of a ribosome. Side and front view.
(A) Small subunit. (B) Large subunit. (1) Head. (2) Platform. (3) Base. (4) Ridge. (5) Central protuberance. (6) Back. (7) Stalk. (8) Front.
Free ribosomes occur in all cells, and also in mitochondria and chloroplasts in eukaryotic cells. Several free ribosomes can associate on a single mRNA molecule to form a polyribosome or polysome. Free ribosomes usually produce proteins that are used in the cytosol or in the organelle they occur in. When certain proteins are synthesized by a ribosome, it can become "membrane-bound", associated with the membrane of the nucleus and the rough endoplasmic reticulum (in eukaryotes only) for the time of synthesis. They insert the freshly produced polypeptide chains directly into the ER, from where they are transported to their destinations Bound ribosomes usually produce proteins that are used within the cell membrane or are expelled from the cell via exocytosis.
Figure 2 : Large (1) and small (2) subunit fit together.
The ribosomal subunits of prokaryotes and eukaryotes are quite similar. However, prokaryotes use 70S ribosomes, each consisting of a (small) 30S and a (large) 50S subunit, whereas eukaryotes use 80S ribosomes, each consisting of a (small) 40S and a (large) 60S subunit.[The unit S means Svedberg units, a measure of the rate of sedimentation of a particle in a centrifuge, where the sedimentation rate is associated with the size of the particle. Svedberg units are not additive - two subunits together can have Svedberg values that do not add up to that of the entire ribosome.]
Figure 3 : Translation (1) of mRNA by a ribosome (2) into a polypeptide chain (3).
The mRNA begins with a start codon (AUG) and ends with a stop codon (UAG). Both ribosomal subunits (small and large) assemble at the start codon (the 5' end of the mRNA). The ribosome uses tRNA[transfer RNAs are RNA molecules that carry an amino acid and present the matching codon, according to the genetic code, to the ribosome.] which matches the current triplet on the mRNA to append an amino acid to the polypeptide chain. This is done for each triplet on the mRNA, while the ribosome moves towards the 3' end of the mRNA. Usually, several ribosomes are working parallel on a single mRNA.
The structure and function of ribosomes, and their attendant molecules, known as the translational apparatus, has been of ongoing research interest since the mid 20th century on through the early 21st century.
See also: protein assembly