Isolated by Palade (1955), ribosomes are the ba¬sophilic granules distributed in the cytoplasm and are rich in RNA. These granular organelles are not enclosed by any membrane. Ribosomes are present both in prokaryotic and eucaryotic cells except in the mam¬malian RBC. These are the smallest cell organelles with varying diameter of about 150-250A and are either found freely in cytoplasm or attached to the endoplasmic reticulum or nu¬clear membrane in eucaryotic cells.
Ribosomes are isolated by differ¬ential centrifugation. And its sedimentation coefficient is measured in Svedberg units (s). On the basis of sedimentation coefficient, two types of ribosomes have been observed: 70s and 80s ribosomes. 70s ribosomes are found in all prokaryotic cells including the chloroplasts and mitochondria of eucaryotic cells. These granules are further subdivided into two subunits¬ the larger 50s subunit holds the smaller 30s one over it like a cap.
The 80s ribosome is larger and is found in eucaryotic cells. It consists of two subunits-60s and 40s. The larger 60s subunit holds the smaller 40s subunit like a cap over it. During protein synthesis many ribosomes occur in a chain on a common m-RNA strand and are called polysomes or polyribosomes. Ribosomes are made up of r-RNA (ri¬bosomal RNA) and proteins; and are the centre of protein synthesis.
The attachment of ribosomes to endoplasmic reticulum takes place through the 60s subunit by electro¬static reactions and also by the nascent polypeptide chain that grows from the ribosome and penetrates across the membrane. The two transmembrane glycoproteins ribophorin-I and ribophorin-II present on endoplasm reticulum are responsible for this.