Ribosomes are very small sub-spherical granular organelles, not bounded by any membrane. They are comprised of ribonucleoproteins and they are the place of protein synthesis. They take place in big number.


 Figure: Ribosome

All ribosome are150-250A in diameter and comprises of two unequal sub units, a big dome shaped and a smaller ovoid one. The smaller sub unit fits over the bigger one similar to a cap. These two sub units take place separately in the cytoplasm and attach to form ribosomes only at the time of protein synthesis. At the time of protein synthesis numerous ribosomes line up and attach an mRNA chain to synthesize numerous copies of a particular polypeptide. Such a string of ribosomes is termed as polysome.


Figure: Polyribosome

Ribosomes take place in cytoplasmic matrix and in certain cell organelles. Accordingly, they are termed as cytoplasmic ribosomes or organelle ribosomes. The organelle ribosomes are found in mitochondria and plastids. The cytoplasmic ribosomes might remain free in the cytoplasmic matrix or joined to the surface of the endoplasmic reticulum. The attached ribosomes usually transfer their proteins to cisternae of endoplasmic reticulum for transport to other portions both inside and outside the cell.

Depending on size or sedimentation coefficient(s), ribosomes are of two kinds: 70s and 80s. 70s type of ribosomes are mainly found in all prokaryotic cells and 80s type are found in eukaryotic cells. S is Svedberg unit that is a measure of particle size with which the particle sediments in a centrifuge. In eukaryotic cells, synthesis of ribosomes takes place within the nucleolus. The ribosomal RNA is synthesized in the nucleolus. The ribosomal proteins are synthesized in the cytoplasm and move to the nucleolus for the creation of ribosomal sub units by complexing with rRNA. The sub units pass out into the cytoplasm via the nuclear pores. In prokaryotic cells, both proteins and ribosomal RNAs are synthesized in the cytoplasm. Therefore the ribosomes operate as the protein factories of the cell.

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