Ribosomes are the protein factories in our cells. The way things work in a cell mean that the DNA codes for specific sequences of amino acids in various proteins, and these proteins, on appropriately folding are responsible for various activities and functions of the cell. Ribosomes do just that, translate the genetic information from the mRNA into proteins. They have a significant role in the translation phase of protein synthesis.
Ribosomes are not organelles in traditional sense. They are basically membrane bound particulate structures which are produced in the nucleus of our cell. They are RNA and protein complexes, composed of about 65% RNA and 35% proteins (Ribonucleo protein or RNP). They are about 20 nano meters (200 Angstroms) in diameter. Structurally, they consist of two sub units which fit together and work as a one functional unit. These subunits are produced and assembled in the nucleus. In certain bacteria, however, ribosomes are created in the cytoplasm of their cells rather than in the nucleus, by translating the operons of multiple ribosome genes. This process is known as Ribosome Biogenesis and this synthesis and processing of the rRNAs and their assembly with proteins involves over 200 proteins all working together.
Ribosomes can exist either in free state or bound, but usually they are found as polysomes or polyribosomes attached to the mRNA. Free ribosomes can freely move in the cytoplasm of the cell. Proteins produced by the free ribosomes are for use within the cell. Membrane-bound ribosomes produce proteins which transmembrane or peripheral membrane proteins and those which are to be sent out of the cell via exocytosis. Basically, the free and bound ribosomes differ only in their intracellular special distribution, their structure and functions are, however, the same. Whether they exist as free or bound depends on the presence of a signal sequence targeting the endoplasmic reticulum on the protein which is being synthesized by them. Ribosomes use a series of codons from messenger RNAs (mRNA) to identify the amino acid sequence required to make the proteins. They then use the mRNA template to attach appropriate amino acids to the codon (which are 3 nucleotides). They obtain these amino acids from the transfer RNA (tRNA) and are known as an anti-codon. Thus, they put together different amino acids to form a chain of polypeptides which forms the proteins.
Unlike other organelles, since they are particulate rather than discrete mass structures, they are measured using the Svedberg unit (S) which is the sedimentation coefficient. Normally, eukaryotic cells present in humans and most other animals have 80S ribosomes, each consisting of a small sub unit about 40S and a larger subunit about 60S.
Reference: Introduction to Anatomy & Physiology, by Tortora and other lecture notes