2. The molecular mechanism of protein synthesis was mainly revealed during the 2nd half of the 20th century Ribonucleoprotein particles(were later called ribosomes) were revealed to be the site of protein synthesis in rat liver cells, using radioactively labeled amino acids and immediate subcellular fractionations(early 1950s, by zamecnik Amino acids were found to be activated by attaching to a special form of heat-stable RNa molecules(later called tRNas before being incorporated into polypeptides(1950s by hoagland and zamecnik)2. The molecular mechanism of protein synthesis was mainly revealed during the 2nd half of the 20th century • Ribonucleoprotein particles (were later called ribosomes) were revealed to be the site of protein synthesis in rat liver cells, using radioactively labeled amino acids and immediate subcellular fractionations (early 1950s, by Zamecnik). • Amino acids were found to be activated by attaching to a special form of heat-stable RNA molecules (later called tRNAs) before being incorporated into polypeptides (1950s, by Hoagland and Zamecnik)