A ribosome is an organelle composed of rRNA (ribosomal RNA)proteins. It translates mRNA into a polypeptide chain (e.g., a protein). It can be thought of as a factory that builds a protein from a set of genetic instructions. Ribosomes can float freely in the cytoplasm (the internal fluid of the cell) or bind to the endoplasmic reticulum, or to the nuclear envelope.
Ribosomes consist of two subunits that fit together (Figure 2) and work as one to translate the mRNA into a polypeptide chain during protein synthesis (Figure 3). Each subunit consists of one or two very large RNA molecules (known as ribosomal RNA or rRNA) and multiple smaller protein molecules. Experiments have shown that the rRNA are the crucial components in protein synthesis, and that one aspect of the process, peptide transfer, can occur in the presence of rRNA alone, albeit at a lower rate. This suggests that the protein components of ribosomes act as a scaffold that may enhance the ability of rRNA to synthesise protein.
The structure and function of the ribosomes and associated molecules, known as the translational apparatus, has been of research interest since the mid 20th century and is a very active field of study today.
Figure 2 : Large (1) and small (2) subunit fit together (note this figure mislabels angstroms as nanometers)
Free ribosomes occur in all cells, and also in mitochondria and chloroplasts in eukaryotic cells. Free ribosomes usually produce proteins that are used in the cytosol or in the organelle they occur in. As the name implies, they are not bound to anything within the cell.
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.
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 bound (large) 60S subunit. However, the ribosomes found in chloroplasts and mitochondria of eukaryotes are 70S. [The unit S means Svedberg units] Also, the 70S ribosomes are vulnerable to some antibiotics that the 80S ribosomes are not. This helps pharmaceutical companies create drugs that can destroy a bacteria infection without harming the animal/human host's cells.
Figure 3 : Translation of mRNA (1) by a ribosome (2) into a polypeptide chain (3). The mRNA begins with a start codon (AUG) and ends with a stop codon (UAG).
In Figure 3, both ribosomal subunits (small and large) assemble at the start codon (the 5' end of the mRNA). The ribosome uses tRNA (transfer RNAs which are RNA molecules that carry an amino acid and present the matching anti-codon, according to the genetic code, to the ribosome) which matches the current codon (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 in bacterial cells, several ribosomes are working parallel on a single mRNA. this is what we call polyribosome or polysome.