Ribosomes are different than the ribozymes mentioned in the question above—they actually serve as the site of translation (see above). This combination of RNA and protein is a meeting place for mRNA and tRNA. Structurally, a ribosome is composed of two parts known as the large and small subunits. Each of these is a combination of protein and a type of RNA: rRNA. At the beginning of translation, the two subunits form a structure around the mRNA molecule as the first tRNA (the one matching the first methionine—also called fMet) arrives. The completed ribosome has niches that hold up to three tRNAs at a time. Because a cell has so many ribosomes at any one time, rRNA is the most common type of RNA found in cells.