There are two types of ribosomes.
- 70S ribosomes: 70S ribosomes are present in primitive cells. It is small, with a diameter of 150Å and a molecular weight of 2.7×106 Daltons. Its two sub units are 50S and 30S. During protein synthesis, the 50S and 30S subunits combine to form the 70S unit in progenitor cells. Bacteria, mitochondria and chloroplasts contain 70S ribosomes.
- 80S ribosomes: 80S ribosomes are present in real cells. Its molecular weight is 40×106 daltons. The two subunits of the 80S ribosome are 60S and 40S. During protein synthesis, the 60S and 40S subunits combine to form the 80S unit in normal cells.
In addition, 77S ribosomes are known to exist in fungi and 55S ribosomes in mammalian mitochondria.