Ribosomes: are minute nonmembranous and dense spheroidal particles or granules. First observed by electron microscopy by palade 1955. Being only about 200 to 250 a in diameter these are smallest, but most abundant of all cytoplasmic organelles. Every cell contains several thousands to millions of ribosomes. Most of these are linked at the surface of endoplasmic reticulum many are free in the cytosol, and some are found in the in the nucleus and mitochondria also in chloroplasts of plant cells.
Structurally. Each ribosome has two parts a proximal larger subunit linked to the endoplasmic are composed of RNA and proteins approximately in equal amounts. The ribosomal RNA ( rRNA) has only four macromolecules . Three in the larger subunit and one the smaller subnuits. Considerable portion of these RNA molecules secondarily become double stranded due to intermolecular the intermolecular base pairing so that the molecules become 3 dimensional .The ribosomal protein include 73 macromolecules 40 in large in 33 in small subunits .
Ribosomal subunits are assembled from their component r RNA and protein molecules in the nucleolus of nucleus. These are of the four r RNA molecules are synthesized in the nucleolus . The fourth is synthesized by some chromosome outside the nucleolus. This and the 73 protein molecules synthesized in the cytoplasm migrate to the nucleolus for being incorporated in ribosomal subunits. The latter diffuse out into the cytoplasm and assemble to form ribosome's most of which link the endoplasmic reticulum.