Ribosomes are small organelles found in the cells of all life forms. They are quite small, only a few hundred nanometers across, and are composed of ribosomal ribonucleic acid (rRNA) and other catalytic proteins. Their main function is to produce a variety of proteins from simple genetic instructions that propagate outwards from the cellular nucleolus in the form of messenger RNA (mRNA). They float in the cytoplasm of a cell or bind to the endoplasmic reticulum, ribbon-like structures found within the cell.
Sometimes, these organelles are referred to as simply RNA. Like DNA, they are long chains of amino acids, but their base pairs are different, and they are usually not as long. Ribosomes play a key part in protein synthesis, the process that generates organic tissue. Genetic instructions for the creation of new proteins come from mRNA. They always have two subunits which interlock and behave as a single entity.
The exact type of ribosome found within a cell can vary based on the kind of organism that the cell is a part of. Eukaryotes (organisms with cellular nuclei) have one kind, whereas prokaryotes (unicellular organisms without nuclei) have another. Certain organelles within the cell, chloroplast and mitochondria, have their own distinct version as well. These organelles make up the majority of RNA content within a cell, about 95%.
In 2001, the entire atomic structure of one ribosome was published in scientific journals, enabling scientists to synthesize it from scratch. This event resulted in considerable controversy and speculation that one day scientists may be able to build living organisms atom by atom.