The ribosomes are small dense, rounded and granular particles. They contain ribonucleoprotein. They occur either freely in the matrix of the mi-tochondria, chloroplast and cytoplasm or remain attached with the membrane of the endoplasmic reticulum and nucleus.
The ribosomes were described by G.E.Palade in 1952. The name ' ribosome' was coined by R. B. Roberts in 1958.
The ribosomes occur in both prokaryotic and eukaryotic cells. In the cells in which active protein synthesis takes place, the ribosomes remain at-tached with the membranes of the endoplasmic reticulum. The cells where such active synthesis happens are pancreatic cells, hepatic cells, osteoblasts, serous cells of submaxillary gland, chief cells of the glandular stomach, thy-roid cells and mammary gland cells.
The ribosomes are spheroid structures with a diameter of 150 to 250 �. Each ribosome is composed of two subunits. One subunit is large in size and has a dome like shape. The other ribosomal subunit is smaller in size and it occurs above the larger subunit forming a cap-like structure.
The ribosomes are chemically composed of RNA and proteins. The ribosomal RNA (rRNA) play a central role in the process of protein syn-thesis. The ribosomal proteins enhance the catalytic function of the rRNA. The functioning of rRNA is under genetic control.