The eukaryotic ribosome (i.e., one not found in mitochondria and chloroplasts) is larger than the prokaryotic 70S ribosome. It is a dimer of the 60S and the 40S subunit, about 22 nm in diameter, and includes the sedimentation coefficient of 80S and a molecular weight of 4 million. Eukaryotic ribosomes can be either associated with the endoplasmic reticulum or free in the cytoplasmic matrix. The ribosome is the primary cell factory and is necessary for all life on Earth. The ribosome is divided into two subunits in complex eukaryotes, such as humans, and is connected with big and small ribosomal RNAs, which create the ribosome's functional core and periphery, respectively. Five more RNA species, as well as a few proteins, are involved in ribosome activity. The tiny nuclear and the bigger and more variable 5S ribosomal RNAs, as well as the tandem small and large non-coding RNAs, U6 and U3, encode ribosomal RNAs.