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

About Author

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Sushmita Baniya

Sushmita Baniya is pursuing her Master’s degree in Medical Microbiology from the National College of Science and Technology (NIST), Kathmandu, Nepal. She did her Bachelor’s degree in Microbiology from Birendra Multiple Campus, Chitwan, Nepal. She is interested in Genetics and Molecular Biology.

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