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Ribosomes Diagram and Functions

Introduction:

  • Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953).
  • This is a cellular particle made of RNA and protein that serves as the site for protein synthesis in the cell.
  • The ribosome reads the sequence of the messenger RNA (mRNA) and, using the genetic code, translates the sequence of RNA bases into a sequence of amino acids.

Detailed Explanation:

  • Ribosomes are the smallest, submicroscopic ribonucleoprotein particles or palade particles or engines of the cell.
  • These are membraneless structures.
  • They are also known as protein factories as they are mainly involved in the synthesis of proteins or polypeptide synthesis.
  • Ribosomes are found in both prokaryotic and eukaryotic cells except mature RBC and mature sperm.
  • These occur in large numbers in eukaryotic cells as compared to the prokaryotic cells.
  • Ribosomes may occur singly or in groups called polyribosomes or polysomes.
  • The different ribosomes of a polyribosome are connected with a thick strand of messenger or mRNA.
  • The maintenance of polyribosomes requires energy.
  • Polyribosomes are formed during periods of active protein synthesis when a number of copies of the same polypeptide are required.
  • Depending upon the place of their occurrence, ribosomes are of two types, cytoplasmic and organellar ribosomes.
  • The organelle ribosomes are found in plastids (plastidoribosomes) and mitochondria (mitoribosomes).
  • Ribosomes are also found in the nucleus which is often known as the director of the cell.
  • The cytoplasmic ribosomes (cytoribosomes) may remain free in the cytoplasmic matrix or attached to the plasma membrane in case of prokaryotes.
  • In eukaryotes, they are either free or attached to the cytosolic surface of the endoplasmic reticulum with the help of a special ribophorin or SRP protein. The attachment occurs through larger or 60 S subunits.
  • Different types of ribosomes may produce different types of proteins, such as structural proteins from free cytoplasmic ribosomes and globular proteins from ribosomes bound to ER.
  • The bound ribosomes generally transfer their proteins to the cisternae of the endoplasmic reticulum for transport to other parts both inside and outside the cell.
  • They are also sent to intracellular organelles like the nucleus, mitochondria and chloroplasts.
  • Newly synthesized proteins are assisted in their folding and transport by specific proteins called chaperones.
  • ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient which is indirectly a measure of density and size.
  • The cytoplasmic and attached ribosomes in eukaryotes are 80 S.
  • The cytoplasmic ribosomes of prokaryotes (PPLO, bacteria, blue-green algae) are 70 S. The organelle ribosomes are also 70 S but in mammalian mitochondria they have a sedimentation coefficient of 55 S.
  • The two subunits of 80 S ribosomes are 60S and 40S while 70S ribosomes have 50S and 30 S subunits.
  • A tunnel occurs between the two subunits for passage of mRNA. The larger subunit has a groove for pushing out the newly synthesized polypeptide.
  • 0.001 M Mg++ concentration is required for the association of two subunits, as a result an intact ribosome is formed.
    30S + 50S = 70S
    40S + 60S = 80S
  • If the concentration of Mg++ increases 10 times then two ribosomes fuse to form a dimer (70S + 70S = 100S dimer, 80S + 80S = 120S dimer)
  • In 70 S ribosomes, the 50S subunit contains the 23S and 5S rRNA and proteins while the 30S subunit contains the 16S rRNA along with proteins.
  • In 80 S ribosomes, the 18S rRNA transcript is incorporated into the small 40S ribosomal subunit, whereas the 5.8S, 28S, and 5S rRNAs are incorporated into the 60S subunit.


    80S-70S
     

The attachment present in the ribosomes are as follows:

(i) mRNA binding site.
(ii) A or aminoacyl site for binding to newly-arrived amino acids carrying tRNA.
(iii) P or peptidyl site which carries growing polypeptide along with tRNA
(iv) E or exit site containing the free tRNA molecule before leaving the ribosome.

Functions of ribosomes:

  • Ribosomes are responsible for the synthesis of proteins or polypeptides.
  • The free ribosomes can synthesize structural and enzymatic proteins whereas the attached ribosomes synthesize proteins for transport.
  • Ribosomes also provide enzymes ( e.g., peptidyl transferase) factors for the condensation of amino acids to form polypeptides.
  • Ribosomes contain ribosomal RNAs that provide the attached points to mRNA and tRNAs.
  • It provides protection to the newly synthesized protein from cytoplasmic enzymes by enclosing it in the groove of a larger subunit till it attains secondary structure.

Differences between the 80S and 70S Ribosomes:
 

80S Ribosomes 70S Ribosomes
They occur only in eukaryotic cells. They occur in prokaryotic cells, mitochondria and chloroplast organelle.
This occurs inside the cytoplasm of the eukaryotes (either free or associated with the ER) . The presence of the ribosomes occurs freely inside the cytoplasm of prokaryotes and the stroma of plastids and matrix of mitochondria organelle.
This type of ribosomes is synthesized in the nucleolus This type of ribosomes are synthesized in the cytoplasm of prokaryotes or matrix of semi autonomous cell organelles.
The two subunits are the 40S and 60 S. The two subunits are the 30S and 50 S.


Frequently Asked Questions (FAQs)

Question 1. State the difference between the 70S and 80S ribosomes?
Solution: Differences between the 80S and 70S Ribosomes:

 

80S Ribosomes 70S Ribosomes
They occur only in eukaryotic cells. They occur in prokaryotic cells, mitochondria and chloroplast organelle.
This occurs inside the cytoplasm of the eukaryotes (either free or associated with the ER) . The presence of the ribosomes occurs freely inside the cytoplasm of prokaryotes and the stroma of plastids and matrix of mitochondria organelle.
This type of ribosomes is synthesized in the nucleolus This type of ribosomes are synthesized in the cytoplasm of prokaryotes or matrix of semi autonomous cell organelles.
The two subunits are the 40S and 60 S. The two subunits are the 30S and 50 S.


Question 2. Mention the function of ribosomes.
Answer: Functions of ribosomes:

  • Ribosomes are responsible for the synthesis of proteins or polypeptides.
  • The free ribosomes can synthesize structural and enzymatic proteins whereas the attached ribosomes synthesize proteins for transport.
  • Ribosomes also provide enzymes ( e.g., peptidyl transferase) factors for the condensation of amino acids to form polypeptides.
  • Ribosomes contain ribosomal RNAs that provide the attached points to mRNA and tRNAs.
  • It provides protection to the newly synthesized protein from cytoplasmic enzymes by enclosing it in the groove of a larger subunit till it attains secondary structure.

Question 3. Which ion is essential for the joining of the two subunits of ribosomes and in what concentration?
Answer:

  • The two subunits of 80S ribosomes are 60S and 40S and the two subunits of 70S ribosomes are 50S and 30S.
  • 0.001 M Mg++ concentration is required for the association of two subunits as a result an intact ribosome is formed.
    30S + 50S = 70S
    40S + 60S = 80S
  • If the concentration of Mg++ increases 10 times then two ribosomes fuse to form a dimer (70S + 70S = 100S dimer, 80S + 80S = 120S dimer)

Question 4. Define sedimentation coefficient.
Answer:

  • ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient which is indirectly a measure of density and size.
  • The cytoplasmic ribosomes of eukaryotes are 80 S.
  • The cytoplasmic ribosomes of prokaryotes (PPLO, bacteria, blue-green algae) are 70 S. The organelle ribosomes are also 70 S but in mammalian mitochondria they have a sedimentation coefficient of 55 S.
  • The two subunits of 80 S ribosomes are 60S and 40S while 70S ribosomes have 50S and 30 S subunits.

Question 5. Mention the subunits and their components of 70 S and 80 S type of ribosomes.
Answer:

  • The two subunits of 80 S ribosomes are 60S and 40S while 70S ribosomes have 50S and 30 S subunits.
  • Ribosomes are responsible for the synthesis of proteins or polypeptides.
  • A tunnel occurs between the two subunits for passage of mRNA. The larger subunit has a groove for pushing out the newly synthesized polypeptide.
  • In 70S ribosomes, the 50S subunit contains the 23S and 5S rRNA while the 30S subunit contains the 16S rRNA.
  • In 80S ribosomes, the 18S rRNA transcript is incorporated into the small 40S ribosomal subunit, whereas the 5.8S, 28S, and 5S rRNAs are incorporated into the 60S subunit.

Question 6. Name the membraneless, protein synthesizing organelle found in both prokaryotes and eukaryotes.
Answer:

  • The membraneless, protein synthesizing organelle found in both prokaryotes(70S) and eukaryotes(80S) are ribosomes.
  • Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953).
  • Ribosomes are the smallest, submicroscopic ribonucleoprotein particles or palade particles or engines of the cell.
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