An organelle in the cytoplasm of a living cell; they attach to mRNA and move down it one codon at a time and then stop until tRNA brings the required amino acid; when it reaches a stop codon it falls apart and releases the completed protein molecule for use by the cell, the ribosome is where protein synthesis takes place.
As I Found it at www.wisegeek.com,Ribosomes are small organelles found in the cells of all life forms. They are quite small, only afew hundred nanometers across. It has been theorized that ribosomeswere once independent life forms, reproducing on their own, until otherchemicals such as carbohydrates and DNA were brought into the fold at a later time. Ribosomes are composed of rRNA (ribosomal RNA, or ribonucleic acid) and other catalytic proteins. Their main function is to produce a variety of proteins from simple genetic instructions which propagate outwards from the cellular nucleolus in the form of messenger RNA (mRNA). Ribosomes float in the cytoplasm of a cell or bind to the endoplasmic reticulum, ribbon-like structures found within the cell.