A. The eukaryotic ribosomes are 80S and prokaryotic ribosomes are 70S.
B. Each ribosome has two sub-units.
C. The two sub-units of 80S ribosome are 60S and 40S while that of 70S are 50S and 30S.
D. The two sub-units of 80S ribosome are 60S and 20S and that of 70S are 50S and 20S.
E. The two sub-units of 80S are 60S and 30S and that of 70S are 50S and 30S.

• (A) A, B, E are true
• (B) B, D, E are true
• (C) A, B, C are true
• (D) A, B, D are true

Detailed Explanation

Ribosomes are known as the "Protein Factories" of the cell. They are granular structures first observed under the electron microscope as dense particles by George Palade (1953).

• Statement A is True: Prokaryotic cells (like bacteria) contain 70S ribosomes, whereas Eukaryotic cells (cytoplasm) contain 80S ribosomes. (Note: Eukaryotic organelles like mitochondria and chloroplasts contain 70S ribosomes).
• Statement B is True: Every ribosome, regardless of its type, consists of two subunits: one large and one small. These subunits only associate during protein synthesis in the presence of $Mg^{2+}$ ions.
• Statement C is True:
  • 80S ribosome = 60S (large subunit) + 40S (small subunit).
  • 70S ribosome = 50S (large subunit) + 30S (small subunit).
  The 'S' stands for Svedberg’s Unit (sedimentation coefficient), which indirectly measures density and size.
• Statements D & E are False: These provide incorrect mathematical combinations for the subunits.

Related Theory: Ribosome Biology

Ribosomes are unique because they are non-membranous organelles found in both prokaryotes and eukaryotes.

1. Structural Composition

Ribosomes are composed of ribonucleic acid (rRNA) and proteins. They do not have a surrounding membrane.

• Prokaryotic Ribosomes (70S): Found in the cytoplasm of prokaryotes. Subunits are 50S and 30S.
• Eukaryotic Ribosomes (80S): Found in the cytoplasm of eukaryotes. Subunits are 60S and 40S.
• Organellar Ribosomes: Mitochondria and Chloroplasts in eukaryotes contain 70S ribosomes, supporting the endosymbiotic theory.

2. The Svedberg Unit (S)

The sedimentation coefficient (S) is a measure of how fast a particle settles in a centrifuge. It is not additive. This is why 60S + 40S equals 80S (not 100S), as the sedimentation rate of the joined complex depends on its combined shape and density rather than just mass.

3. Functional Importance

• Translation: They are the site where mRNA is translated into a polypeptide chain (protein).
• Polyribosomes/Polysomes: Several ribosomes may attach to a single mRNA strand to form a chain, allowing multiple copies of the same protein to be synthesized simultaneously.

Comparison Table

Frequently Asked Questions (FAQs)