Ribosome- Definition, Types, Structure, Composition, Functions

What is Ribosome?

  • The ribosome is a large complex that is made from dozens of small proteins.
  • These small proteins are ribosomal proteins.
  • It consists of many several RNA molecules which are called ribosomal RNAs (rRNAs).
  • The word ribosome is made up of ribo+somes.
  • “Ribo” stands for ribonucleic acid.
  • In the Greek word, ‘soma’ refers to the body. From it, the “somes” is derived.
  • Occurrence of ribosomes:
    • in the prokaryotic cells:  freely in the cytoplasm
    • in the eukaryotic cells: freely in the cytoplasm or remain in the endoplasmic reticulum in the outer surface.
  • By differential centrifugation, the ribosomes can be isolated from the cell.
  • By the different optical and electronic techniques the sedimentation coefficient of the ribosomes can be determined which is expressed as Svedberg (S).
  • The eukaryotic cell in its cytoplasm contains millions of ribosomes.
  • They are similar in structure and function in eukaryotic and prokaryotic cells.
  • They are made up of :
    • one large subunit
    •  one small subunit,
  • It fits each other and forms a complete ribosome.
  • It has a mass of several million daltons.
  • The small ribosomal subunit matches the codons of the mRNA which is present in the tRNAs.
  • Amino acids are covalently linked by the peptides bond.
  • Then the polypeptide chain is formed.
  • The larger and smaller subunits come together on an mRNA molecule near its 5′ end. Then protein synthesis is started.
  • the mRNA moves forward in a 5′-to-3′ direction.
  • Then the ribosome translates its nucleotide sequence into an amino acid sequence, one codon at a time.
  • It will use the tRNAs as adaptors.
  • At the place of the growing polypeptide chain., each amino acid is thereby added in the accurate sequence.
  • There are two different subunits of the ribosome which separate after the protein formation.
  •  In every second, 2 amino acids are added.
  • The ribosome of the bacteria performs faster than the eukaryotic ribosome.
  • In the case of the bacterial ribosome, adding about 20 amino acids in one second.
Ribosome
Figure: Ribosome. Created with BioRender.com

Types of Ribosomes

Based on the size and the sedimentation coefficient (S), ribosomes are of two types: 

  • 70S ribosome
  • 80S ribosome

70S ribosome

  • They are smaller in size.
  • Sedimentation coefficient: 70S 
  • Molecular weight:  2.7× 106 daltons.
  • They are found in:
    • prokaryotic cells of the blue-green algae and bacteria.
    • mitochondria and chloroplasts of eukaryotic cells.

80S ribosome

  • Sedimentation coefficient: 80S 
  • Molecular weight:  40 × 106 daltons. 
  • They are found in the eukaryotic cells i.e. in plants and animals.
  • The ribosomes present in mitochondria and chloroplasts are smaller than 80S cytoplasmic ribosomes.
  • In the 80S ribosome of yeast, 79r-protein are present where only 12 r-protein are found to be specific.

Structure of Ribosome

  • Ribonucleoprotein means it consists of RNA and proteins.
  • In it, RNA is 32 to 62%.
  • Others are protein.
  • Its diameter is 150 to 250 A° 
  • It is porous and hydrated.
  • It is consists of two subunits:
    • Larger  subunit: dome-shaped
    • Smaller subunit:  cap-like.
  • 50S and 30S subunits are present in the 70S ribosome.
    • 50S: larger subunit, size:  160A° to 180 A°
    • 30S: smaller subunit.
  • 60S and 40S ribosomes are present in the 80S subunits.
    • 60S : larger subunit
    • 40S: smaller subunit
  • Due to the greater concentration of the Mg++(.001M) ions, these subunits are attached.
  • The dimer is also formed in this higher concentration.
  • The ribosomal subunits will get detached when there is a decrease in the concentration of Mg++ions in the matrix.
  • These two ribosomes are known as monosomes.
  • Later, due to the aggregation of a large number of ribosomes, there is the formation of the polyribosomes or polysomes.

Chemical Composition

  • Ribosomal RNAs
  • Ribosomal proteins
  • Metallic ions 

Ribosomal RNAs

  • 70S ribosomes consist of three types of rRNA:
    • 23S rRNA
    • 16S rRNA
    • 5S rRNA
  • In the 50S ribosomal subunit (larger subunit), 23S and 5S rRNA are present.
  • In the 30S ribosomal subunit, the 16S rRNA is present.
  • In the 80S ribosomes  four types of rRNA are present:
    • 28S rRNA
    • 18S rRNA
    • 5S rRNA
    • 5.8 rRNA
  • In the larger 60S ribosomal subunit, 28S, 5S, and 5.8S rRNAs are present.
  • There is the presence of 18S rRNA in the 40S ribosomal subunit ( Smaller)

Ribosomal proteins

  • Bacteria are composed of different ribosomal proteins.
  • It was found that E. coli consists of 55 ribosomal proteins.
  • Example; Core proteins (CP), Split proteins (SP)  

Metallic ions

  • divalent metallic ions:Mg++, Ca++ and Mn++ 

Functions of Ribosome

  • Ribosome plays an important role during the biosynthesis of protein. This process is known as translation.
  • Ribosomes functions as catalysts during peptidyl transfer and peptidyl hydrolysis. 
  • Ribosome protects the mRNA strand from the nuclease enzyme. The mRNA during the translation process lies in between the larger and smaller subunit of the ribosome.
  • The nascent polypeptide chain is protected from the activity of protein digestive enzymes.

How does the ribosomal movement take place in translation?

  • In addition to a binding site for an mRNA molecule, it consists of other 3 binding sites. for tRNA molecules;
    • A site
    • P site
    • E site
  • Amino acid needs to be added to a growing peptide chain.
  • The charged tRNA whose base pairs with the complementary codon on the mRNA molecule enters the A site.
  • Then in the new forming polypeptide chain, an amino acid is added which is held by the tRNA in the adjacent  P site. 
  • Then the large ribosomal subunit moves forward to the E site.
  • This process or the cycle is repeated.
  • Each time an amino acid is added to the polypeptide chain, where the new protein grows from its amino to its carboxyl end.
  • Finally, the stop codon will be encountered in the mRNA.
  • The termination release factors like the RF1 and RF2 recognize the stop codons.
  • Then the peptidyl-tRNA bond is hydrolyzed.
  • Finally, the newly formed polypeptide is released from the ribosome.
  • By the study of the structure and its biochemical characteristics, antibacterial agents are developed in such a way they can inhibit this protein synthesis process.
  • Examples of  such antibiotics are:
    • Aminoglycosides
    • Chloramphenicol
    • Fusidic acids
    • Lincosamides
    • Macrolides
    • Oxazolidinone
    • Streptogramins 
    • Tetracyclines
Antibiotics targeting 50S subunit Antibiotics targeting 30S subunit
Blasticidin S Doxycycline
Chloramphenicol Tigecycline
Clindamycin Edeine
Lincomycin Kasugamycin
Dalfopristin (SA) Paromomycin
Quinupristin (SB) Neomycin
Erythromycin Spectinomycin
Telithromycin Streptomycin
Evernimicin
Avilamycin Antibiotic targeting EF-G
Linezolid Fusidic acid
Puromycin
Sparsomycin Antibiotic targeting EF-Tu
Thiostrepton Kirromycin
Tiamulin

References

  • Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S.Chand and Company Ltd.
  • Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.
  • Wilson, D. N. (2014). Ribosome-targeting antibiotics and mechanisms of bacterial resistance. Nature Reviews Microbiology, 12(1), 35–48. https://doi.org/10.1038/nrmicro3155 
  • Wilson, D. N., & Cate, J. H. D. (2012). The structure and function of the eukaryotic ribosome. Cold Spring Harbor Perspectives in Biology, 4(5), 5. https://doi.org/10.1101/cshperspect.a011536
  • https://www.notesonzoology.com/cytology/ribosome-meaning-types-and-structure/2174

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