Ribosomes are small organelles found in the cells of all life forms. They are quite small, only a few hundred nanometers across. It has been theorized that ribosomes were once independent life forms, reproducing on their own, until other chemicals 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.

Ribosomes are sometimes 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 as DNA. 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. Ribosomes 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 (multi-celled organisms) have their own kind of ribosome, whereas prokaryotes (single-celled organisms) have another. Certain organelles within the cell, chloroplast and mitochondria, have their own distinct ribosomes as well. Ribosomes make up the majority of RNA content within a cell, being responsible for about 95%.

In 2001, the entire atomic structure of one ribosome was published in scientific journals, enabling scientists to synthesize the particle from scratch. This event resulted in considerable controversy and speculation that one day scientists may be able to build living organisms atom by atom.