RIBOSOME STRUCTURE, r-RNA AND BIOSYNTHESIS
Ultrastructure of Ribosomes
Ribosomes are round, granular and membraneless cell organelle which are chemically nucleoprotein and found enormously in all the prokaryotic and eukaryotic cells. These were discovered first by Claude in 1943 and were named as ‘microsomes’. Robinson and Brown isolated ribosomes from root cells of broad bean. Palade coined the term ‘ribosomes’ and isolated it from animal cells. After his name ribosomes are laso called ‘Palade granules.’ Ribosomes may be defined as “The smallest known electron microscopic, ribonucleoprotein particles attached the on RER or floating freely in the cytoplasm and are the sites of protein biosynthesis”.
OCCURRENCE: Ribosomes are generally found in all known prokaryotic and eukaryotic organisms except mature RBCs. In prokaryotes these are found only in free form in the cytoplasm while in eukaryotes these are found both in the cytoplasm and on the surface of RER. The former is called cytoplasmic and later is called bound form of ribosomes. These may also be found on the surface of nuclear membrane. Some organelles like mitochondria and chloroplast contain ribosomes in the matrix. These are called organellar ribosomes and reffered as ‘mitoribosomes’ and ‘plastidoribosomes’. The ribosomes found on the surface of RER is bound with the membrane with special proteins called ribo-phorines.
Number: The number of ribosomes in a cell depends on the content of RNA. These are more in number in metabolically active cells like plasma cells, livercells, nissl’s granules of nerve cells, meristemati c cells, cancer cells, endocrine cells etc. In a cell of E.coli, the number of ribosomes vary from 10,000-20,000.
Structure: Ribosomes are globular structures having the diameter of 150-250 Å. Each ribosome is made up of two subunits one is smaller and another is larger in size. The later in dome shaped and is covered by cap like smaller unit. In 70S type of ribosome the larger and smaller units are 50S and 30S type. On the other hand, in 80S type, these are of 60S and 40S type, respectively. The two subunits of ribosomes are freely distributed in the cytoplasm. The two subunits unite to form a complete ribosome. Likewise, the two subunits dissociate with each other when the concentration of Mg++ ion decreases in the cytoplasm.
During protein synthesis many ribosomes become attached with mRNA forming a peculiar structure called polyribosome.
Ultrastructure of Ribosomes
The last point about the ultra structure of ribosomes has not been said till date. The credit of giving the present knowledge of the ultrastructure of ribosomes goes to Nauninga. According to him, the size of larger (50S) subunit of 70S type of ribosome is 160 to 180 Å which is pentagonal in shape. This unit has a groove of 40-60 Å size in which the smaller subunit is attached during association. The smaller subunit has a platform, cleft, head, base and also a binding site for nRNA. The smaller unit of 70S and 80S type of ribosomes does not have a definite shape. Florendo in 1968reported a pore like transparent area on the larger unit of 50S of 70S ribosomes.
In between two subunits of ribosomes, mRNA is found. t-RNA molecule is found in the side of nRNA. The new-formed polypeptide chain being synthesized on the ribosome mRNA complex has been seen passing through the transparent pore on the larger unit.
It also has a protuberance, a ridge and stalk. Two binding sites, peptidyl and amino acyl sites are found on the larger units.
The 50S and 30S subunits have been reported to have the molecular weight of 1.8 X 106 Daltons and 0.9 X 106 Daltson, respectively. It must be noted here that size and type of ribosomes and their subunit are determined on the basis of their sedimentation coefficient.
Types of Ribosomes
On the basis of their sedimentation coefficient, ribosomes have been classified into two main types:
70S ribosomes:
These are found in prokaryotes and mitochondria and plastids of eukaryotes. Each 70S
ribosome is about 200-290 Å in size and 2.7 X 106 Daltons in its mol. Weight. It
consists of two subunits of 50S and 30S size. Both of these units are made up of
ribosomal RNA and ribosomal proteins. The 50S subunits again consist of 23S and 5S
rRNA and 30 types of proteins. Similarly, the smaller unit is made up of 16S type of
rRNA and 20 types of proteins.
80S ribosomes:
These are the characteristic of eukaryotic cells and found. In their cytoplasm. It consists
of two subunits. The size of larger subunits is 60S and that of smaller subunit is 40S. It
is also made up rRNA and proteins. The 60S subunits consists of 28S rRNA, 5.8 S
rRNA and 5S rRNA and about 50 types of proteins. The smaller subunits is similarly
made up of 18S rRNA and 30 different proteins.
Polyribosomes or Polysomes: When many ribosomes are attached to same mRNA strand, it is called polyribosomes or polysome.
It is formed when a simple protein is required in high quantity. The number of ribosomes in a polysome depends on the length of mRNA. The distance between two adjacent ribosome is about 90 nucleotids.
Origin of ribosomes:
We have studied that ribosomes are solely made up of rRNA and proteins. The former is formed inside the nucleus and the later is produced in the cytoplasm. Therefore, these are partly nuclear and partly cytoplasmic is nature. However, in prokaryotes, since there in no nucleus, ribosomes are totally cytoplasmic in nature.
Functions of Ribosomes.
I. Ribomes are called factories of proteins or engineers of the cell because these are the side of protein synthesis.
II. Sometimes rRNA of ribosomes has been found to function as enzymes controlling the cellular functions. These are called ribozymes.
III. The process of translation of genetic language into the language of enzymes or protein take place at ribosomes. It takes place with the help of nRNA, which, is produced during transcription of nuclear DNA.
IV. In general the ribosomes bound on RER synthesise enzymes for extracellular use e.g., pancreatic cells, chief cells of gastric glands, liver cells etc.
V. Ribosomes temporarily store proteins.
VI. Ribosomes keep the mRNA molecules functionally alive.
RIBOSOMAL RNA or rRNA
The RNA, which is found in ribosomes, is called ribosomal RNA. Ribosomes are chemically ribonucleoprotein as they consist of RNA and proteins. It is known as soluble RNA. Its quantity in a cell is much higher than that of mRNA and tRNA. It constitutes about 80% of total RNA.
On the basis of their sedimentation coefficient or rate of sedimentation, rRNA molecules may be classified into following categories:
1) 28S-rRNA: It has molecular weight more than 10,00000. Sedimentation coefficient is between 21S and 29S. It is found in 60S subunit of eukaryotic ribosomes.
2) 18s-rRNA: It molecular weight is less than a millions. Sedimentaion varies between 12S to 18S. It is found in 40S subunit of ribosomes.
3) 5S-rRNA: It has much lower molecular weight and is found in 30S unit of ribosomes.
Structure of rRNA
Ribosomal RNA molecules are single stranded but in the solution of high ionic concentration, irregular spiral coiling of rRNA is formed. As the ionic concentration of the solution increases, the degree of irregular coiling of rRNA also increases. In this coiling the intramolecular bases show base pairing. The pairing is normal as A pairs with U and C pairs with G.
Function of rRNA
The main function of rRNA may be summarized as below:
In many viruses specially in plant viruses, RNA function as genetic material and carry genetic information from generation to generation.
Different RNAs function as structural component of a cells
mRNA are the site of protein synthesis where polymerization of amino acids takes place through peptide bond formation between amino acid molecules during translation process.
A tRNA molecule has anticodon site and has the capacity of attachment with the complementary condon on mRNA. tRNAs further carry activated amino acids to the mRNA and catalyse peptide bond formation between two amino acid molecules.
Ribosomal RNA(rRNA) is the constituent unit of ribosomes.
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