RIBOSOME STRUCTURE, r-RNA AND BIOSYNTHESIS
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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