What is the function of Ribosomes? What is the types & function of Ribosomes? Full explanation of composition, types & function of ribosomes ?
In this article we will discuss about the Ribosomes:-explanation, composition, types of ribosomes, function.
Explanation of ribosomes
Composition of ribosomes
Types of ribosomes
Function of ribosomes
Ribosomes are minute, granular, and spheri cal bodies found in both prokaryotic and eukaryotic cells. Palade (1955) first observed ribosomes attached to the outer surface of endoplasmic reticulum. The name ribosome was proposed by Haguenau and Roberts in 1958. They are the most abundout cyto plasmic organelles ranging from several thousands to many millions. Under electron microscope ribo somes appear as spheroidal, electron-opaque par ticles. The size of ribosomes vary from 150 Aº to 200 Aº. In eukaryotic cells ribosomes are freely scat tered in the cytosol or they may be found attached to the outer surface of ER. Some mitochondria and chlo roplasts may also contain ribosomes.
The wall of ribosome is non-membranous i.e., it is not made on the unit membrane principle (protein-lipid protein). Chemically, ribosomes are made up of equal amounts of RNA and proteins, thus the name particles of ribonucleoproteins.
Types of ribosomes:
Two types of ribosomes have been observed on the basis of their sedimentation coefficient.
1. 70 S ribosome: These are comparatively smaller in size and mostly found in prokaryotic cells, mitochondria, and chloroplasts. The 70 S ribosomes are composed of two subunits, (i) the 50 S and the (ii) 30 S. The 50 S subunit is larger in size and dome shaped whereas the 30 S subunit is smaller in size and cap-shaped.
2. 80 S ribosome: These are conspicuous in the cytoplasm of eukaryotic cells. The 80 S ribosomes are made of two subunits, the larger, basal 60 S subunit and the smaller, distal 40 S subunit. The 60 S subunit provides site for attachment with the membrane of the ER while the 40 S subunit forms a cap-like covering over the 60 S subunit.
The 60 S subunit further consists of one molecule each of 28 S rRNA, 18 S rRNA, 5 S rRNA and about 50 different kinds of proteins. On the otherhand, the 40 S subunit consists of one 18 S rRNA and 30 different proteins. The two ribosomal subunits remain separate by a narrow groove. Each ribosome has two active sites, the P-site or polypeptide site and the A-site or amino acid site. The A-site is for amino acid tRNA complex and the P-site is for the growth of polypeptide chain.
Usually both the subunits remain separate and condense at the time of protein synthesis. The RNAs and the proteins are held together by electrostatic bonds in the presence of divalent ions.
1. Ribosomes are intracellular sites of protein synthesis. Hence, they are appropriately called the ‘protein factory’ of the cell.
2. The ribosomes act as binding sites for m-RNA. About 6 to 8 ribosomes move along the m-RNA molecule (carrying genetic information) forming a chain or string of ribosomes called poly ribosomes or ergosomes or simply polysomes.
3. Hemoglobin of young blood cell is synthesized in the ribosomes who occur freely in the cytoplasm.
4. The amino acids are transported to the ribosomes for the growth of the polypeptide chain during protein synthesis.