Synonyms:
Ribonucleoprotein (RNP) factories, Palade particles, protein factories, Claude’s particles. As ribosomes are found in mitochondria and chloroplast, they are called organelle within organelle.
Definition:
Ribosomes are sub-microscopic, smallest, dense, membrane-less granular ribonucleoprotein organelles found in all living cells.
History:
A. Claude (1941), first observed ribosomes and called them as microsomes which were actually fragments of RER.
Robinson and Brown (1953) first discovered ribosomes, in plant cells (roots of Vicia).
Palade (1955) isolated ribosomes from animal cells and detected RNA in them.
R.B. Roberts (1958) coined the term ribosome.
Distribution and Number:
The ribosomes are present in both prokaryotic and eukaryotic cells but absent in mature RBC and sperm. In prokaryotic cells, they are found freely scattered in the cytoplasm, but in eukaryotic cells they occur free in the cytoplasmic matrix and also attached to the outer surface of the rough endoplasmic reticulum and nuclear envelope. The ribosomes are also found in the matrix of mitochondria and the stroma of plastids in the eukaryotic cells. These ribosomes are called organeller ribosomes to distinguish them from the cytoplasmic ribosomes.
Ribosomes occur singly (monosomes) or in cluster (polysomes). At the time of protein synthesis 6-8 ribosomes temporarily join with a mRNA to form a cluster called poly ribosome or polysome or ergosome.The number of ribosomes in a cell depends upon the active protein synthesis. In eukaryotic cells up to 10 million present. A plant cells may contain up to 5, 00,000 ribosomes, 10,000 – 30,000 ribosomes, forming 25% of total mass of the bacterial cell.
Types:
On the basis of sedimentation coefficient, measured in Svedberg Units or S units two types of ribosomes have been recognized – 70 S ribosomes and 80 S ribosomes. However, ribosomes with different sedimentation value found in different phyla, e.g. 77S in fungal mitochondria and 55S in mammalian mitochondria.
1.70 S Ribosomes:
These types of ribosomes are found in prokaryotic cell such as bacteria and cyanobacteria, mitochondria and chloroplasts of eukaryotic cells. Their sedimentation coefficient is 70 S and molecular weight 2.7 x 106 Daltons. Each 70S ribosome is made up of two subunits the smaller 30 S subunit remains attached with larger SOS subunit like a cap.
2. 80 S Ribosomes:
They are larger in size than 70S ribosomes. Their sedimentation coefficient is 80 S, and molecular weight 40 x 106 Daltons. Like 70 S ribosomes, it is also made up of two subunits – 60 S and 40 S; with 40 S placed over 60 S subunit.
Ultrastructure:
Ribosomes are smallest and most abundant organelles of a cell. Each ribosome is porous, hydrated and composed of two unequal sub-units, larger one dome- shaped and the smaller one oblate – ellipsoid. The large subunit has a protuberance, a ridge and a stalk. The smaller subunit has a platform, cleft, head and base. It is about half the size of larger subunit. The smaller subunit fits over the larger one at one end like a cap (Fig. 3.38).
The two subunits usually remain separated and come together only at the time of protein synthesis. For the union of two subunits require 0.001M of Mg2+ subunits dissociated below it. When Mg 2+ concentration is above 0.0001M non-functional dimmers are formed. Each ribosome has four sites for specific functions in protein synthesis.
They are:
(i) mRNA binding site in smaller sub-unit
(ii) A-site or amino acyl-tRNA site,
(iii) P-site or peptidyl-tRNA site and
(iv) E-site or exit site to which uncharged t-RNA come before leaving the ribosome (Fig. 3.39).
“Chemically ribosomal,” subunit consists of highly folded ribosomal RNA, (rRNA) and many attached proteins. The ratio of rRNA to protein in prokaryotic and eukaryotic ribosomes is 60:40 and 50:50 by weight respectively. The ribosomal proteins maybe basic, structural or enzymatic in function. The larger subunit of ribosome contains an important enzyme – peptidyl transferase, which brings about the formation of peptide bond. Inside the ribosome, the rRNA remains fully covered with proteins. The ribosomes are therefore, ribonucleoprotein particles (RNP).
Functions:
(a) As protein factories:
The ribosomes are the site of protein synthesis and also provide necessary enzymes for the same. Hence these are called “Protein Factories”.
(b) Free and attached Ribosomes:
Free ribosomes synthesis structural and enzymatic proteins for use inside the cell. The attached ribosomes synthesize proteins for transport (i.e. transport proteins).
(c) Enzymes and Factors:
Ribosomes provide enzymes (e.g. peptidyl transferase) and factors for condensation of amino acids to form polypeptide.
(d) rRNA:
Ribosome contains rRNAs for providing attachment points to mRNA and tRNAs (transfer RNA).
(e) mRNA.
Ribosomes has tunnel for mRNA so that it can be translated properly.
(f) Protection.
A newly synthesized polypeptide is provided protection from cytoplasmic enzymes by enclosing it in the groove of larger subunit of ribosome till it attains secondary structure.