Difference Between Prokaryotic and Eukaryotic Ribosomes

The main difference between prokaryotic and eukaryotic ribosomes is that the prokaryotic ribosomes are small, 70 S ribosomes whereas the eukaryotic ribosomes are larger, 80S ribosomes. Furthermore, prokaryotic ribosomes occur free in the cytoplasm while eukaryotic ribosomes are generally bound to the outer membrane of the nucleus and the endoplasmic reticulum.

Prokaryotic and eukaryotic ribosomes are the cellular machines that facilitate the translation of mRNA into an amino acid sequence. Both types of ribosomes are made up of a large and a small subunit. Each subunit is made up of rRNA and ribosomal proteins.

Key Areas Covered

1. What are Prokaryotic Ribosomes
     – Definition, Subunits, Role
2. What are Eukaryotic Ribosomes
     – Definition, Subunits, Role
3. What are the Similarities Between Prokaryotic and Eukaryotic Ribosomes
     – Outline of Common Features
4. What is the Difference Between Prokaryotic and Eukaryotic Ribosomes
     – Comparison of Key Difference

Key Terms

Eukaryotic Ribosome, Large Subunit, Prokaryotic Ribosome, Small Subunit, Translation

What are Prokaryotic Ribosomes

Prokaryotic ribosomes are the small ribosomes involved in translation, mainly in bacteria and archaea. The large subunit of the prokaryotic ribosomes is 50S while the small subunit is 30S. The structure of large and small subunits of prokaryotic ribosomes are described in the following table.

Prokaryotic Ribosomes (E. coli)

Figure 1: Ribosome Structure
1 – Large subunit, 2 – Small Subunit

However, mitochondria and chloroplasts in the eukaryotic cells also contain 70S ribosomes since these organelles are evolved from endosymbiosis.

What are Eukaryotic Ribosomes

Eukaryotic ribosomes are the large ribosomes that only occur in the eukaryotic cells such as animals, plants, fungi, and other unicellular, eukaryotic organisms. Their large subunit is 60S and the small subunit is 40S.

Eukaryotic Ribosomes (R. norvegicus)

Figure 2: Ribosome Function

Most eukaryotic ribosomes are membrane-bound but, some are free in the cytoplasm. The main function of a ribosome is to serve as a site for protein synthesis by facilitating the linkage of amino acids in an order specified by the mRNA. This process is known as translation. Moreover, ribosomes are also called translational apparatus.

Similarities Between Prokaryotic and Eukaryotic Ribosomes

• Prokaryotic and eukaryotic ribosomes are cellular structures that facilitate the decoding of a codon sequence in a mRNA into an amino acid sequence of a functional protein.
• Both types of ribosomes occur in the cytoplasm.
• They are made up of a large and a small subunit. Each subunit is a ribonucleoprotein complex made up of rRNA and ribosomal proteins.
• The small subunit of prokaryotic ribosomes consists of a single rRNA.

Difference Between Prokaryotic and Eukaryotic Ribosomes

Definition

Prokaryotic ribosomes refer to free ribosomes in prokaryotes while eukaryotic ribosomes refer to large ribosomes that facilitate translation in eukaryotes.

Found in

Prokaryotic ribosomes are found inside bacteria and archaea while eukaryotic ribosomes are found in animals, plants, fungi, and other unicellular eukaryotes with a nucleus.

Size

Prokaryotic ribosomes are small while eukaryotic ribosomes are larger. Further, the mass of a prokaryotic ribosome is 27000 kd while the mass of eukaryotic ribosomes is 42000 kd.

Sedimentation Coefficient

The sedimentation coefficient of prokaryotic ribosomes is 70S while the sedimentation coefficient of eukaryotic ribosomes is 80S.

Diameter

The diameter of a prokaryotic ribosome is ~200 Å while the diameter of a eukaryotic ribosome is ~250-300 Å

Subunits

Prokaryotic ribosomes are made up of 50S and 30S subunits while eukaryotic ribosomes are made up of 60S and 40S subunits.

Number of rRNA in the Molecules in the Large Subunit

The large subunit of prokaryotic ribosomes is made up of two rRNA molecules: 23S rRNA and 5S rRNA while the large subunit of eukaryotic ribosomes is made up of three rRNA molecules; 28S rRNA, 5.3S rRNA, and 5S rRNA.

rRNA to Ribosomal Protein Ration

The prokaryotic ribosomes are made up of 60% rRNA and 40% ribosomal proteins while the eukaryotic ribosomes are made up of 40% rRNA and 60% ribosomal proteins.

Location

Prokaryotic ribosomes occur free in the cytoplasm while most eukaryotic ribosomes are attached to the outer surface of the nucleus and the endoplasmic reticulum.

Conclusion

Prokaryotic ribosomes are bacterial ribosomes that are small (70S) while eukaryotic ribosomes are large ribosomes (80S). Prokaryotic ribosomes occur free in the cytoplasm while most eukaryotic ribosomes are membrane-bound. Both types of ribosomes consist of two subunits called large and the small subunit. The main difference between prokaryotic and eukaryotic ribosomes is the size and the location of ribosomes in the cell.

Reference:

1. Berg, Jeremy M. “Eukaryotic Protein Synthesis Differs from Prokaryotic Protein Synthesis Primarily in Translation Initiation.” Advances in Pediatrics., U.S. National Library of Medicine, 1 Jan. 1970, Available Here

Image Courtesy:

1. “Ribosome structure svg” By Dominus – (CC BY-SA 3.0) via Commons Wikimedia   
2. “Ribosome Translation” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia