Ribosomes are small subspherical granular organelles, not enclosed by any membrane. They are composed of ribonucleoproteins and they are the site of protein synthesis. They occur in large number. Each ribosome is 150-250A in diameter and consists of two unequal sub units, a larger dome shaped and a smaller ovoid one. The smaller sub unit fits over the larger one like a cap. These two sub units occur separately in the cytoplasm and join to form ribosomes only at the time of protein synthesis.
At the time of protein synthesis many ribosomes line up and join an mRNA chain to synthesise many copies of a particular polypeptide.
Such a string of ribosomes is called polysome. Ribosomes occur in cytoplasmic matrix and in some cell organelles. Accordingly, they are called cytoplasmic ribosomes or organelle ribosomes.
The organelle ribosomes are found in plastids and mitochondria. The cytoplasmic ribosomes may remain free in the cytoplasmic matrix or attached to the surface of the endoplasmic reticulum.
The attached ribosomes generally transfer their proteins to cisternae of endoplasmic reticulum for transport to other parts both inside and outside the cell. Depending upon size or sedimentation coefficient(s), ribosomes are of two types. 70s and 80s. 70s type of ribosomes are found in all prokaryotic cells and 80s type are found in eukaryotic cells. S is Svedberg unit which is a measure of particle size with which the particle sediments in a centrifuge. In eukaryotic cells, synthesis of ribosomes occurs inside the nucleolus.
Ribosomal RNA are synthesized in the nucleolus. The ribosomal proteins are synthesized in the cytoplasm and shift to the nucleolus for the formation of ribosomal sub units by complexing with rRNA. The sub units pass out into the cytoplasm through the nuclear pores. In prokaryotic cells, both ribosomal RNAs and proteins are synthesized in the cytoplasm. Thus the ribosomes act as the protein factories of the cell.