Ribosomes join amino acids to synthesize proteins for the cell. Free ribosomes exist in the cell's cytoplasm, and bound ribosomes are part of the rough endoplasmic reticulum.
Ribosomes create proteins through the translation of messenger RNA, or mRNA. Like DNA, mRNA contains special molecules, nucleotides, that code amino acids; three nucleotides translate into one amino acid. Ribosomes move along a strand of mRNA, reading and translating these nucleotides into the appropriate amino acids and linking the amino acids to form proteins.
The ribosome itself is made of two parts, small and large subunits; ribosomal subunits do not come together until it is time to translate mRNA. The small subunit recognizes the nucleotides adenine, uracil and guanine, in that order. It attaches to this section of the mRNA and signals the large subunit to join it and begin translation.
Free ribosomes within the cell's cytoplasm create proteins for use within the same cell. Some ribosomes bind to the membrane of the rough endoplasmic reticulum, a cellular organelle that specializes in protein synthesis. Bound ribosomes create secretory proteins; these proteins travel to other parts of the cell or other cells entirely. Antibodies and insulin are two important secretory proteins that originate from bound ribosomes.