Ribosomes are protein manufacturers within cells. Huge molecules of DNA, or deoxyribonucleic acid, coiled within the chromosomes of every living organism use a universal language called the genetic code. Employed by all cells in the same fashion, the information encoded in DNA acts as a set of instructions for the synthesis of vital protein molecules. Cells assemble thousands of different kinds of proteins using the information within DNA. To construct an analogy, if a single cell were a kitchen, DNA would be a master cookbook and protein molecules would be the meals prepared using the cookbook. In this cellular kitchen, then, ribosomes are the molecular chefs.
The protein molecules made are not directly constructed from DNA. They are synthesized by ribosomes, which use messenger ribonucleic acid (mRNA) molecules as guides. Constructed by copying portions of DNA in chromosomes, mRNA molecules are able to leave the nucleus of the cell and go to the site of protein synthesis in the cytosol (or cytoplasm). Once in the cytosol, the process of interpreting the recipe of DNA into protein involves two phases. The first is called transcription. Transcription creates the mRNA copy of a gene to be expressed. The process is like creating many photocopies of a portion of DNA that can then be sent elsewhere in the cell.
The second process, called translation, directly involves ribosomes, which interpret the "photocopied" information of mRNA molecules. Like barcode scanners in grocery store check-out registers that interpret the black and white UPC code bars of products, ribosomes "read" nucleotide sequences of mRNA and construct protein molecules from amino acids using the encoded information.
Ribosomes are composed of two parts, a large subunit and a small subunit. Additionally, ribosomes contain a distinct kind of RNA found only in ribosomes, called ribosomal RNA (rRNA). During translation, the two separate subunits of a ribosome clasp around a single mRNA molecule. As the ribosome reads the information, it slides along the length of the mRNA molecule until it reaches the end and drops off, leaving the finished protein product. Messenger RNA molecules that have many ribosomes attached to them simultaneously, called polysomes, are formed when multiple protein products are produced from the same mRNA molecule. Ribosomes are found existing free within the cytosol, or as attached structures of the rough endoplasmic reticulum, the organelle which modifies and refines non-functional proteins into functional ones.