Question

The subunits of 80S ribosome include

• A. 40S and 50S
• B. 40S and 60S
• C. 30S and 50S
• D. 30S and 60S

Hint:

Ribosomal sedimentation coefficients are not additive. For eukaryotes: $40\text{S} + 60\text{S} = 80\text{S}$. For prokaryotes: $30\text{S} + 50\text{S} = 70\text{S}$.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Ribosomes are cellular ribonucleoprotein complexes responsible for protein synthesis.
They exist in two main size classes: 70S ribosomes (found in prokaryotes, mitochondria, and chloroplasts) and 80S ribosomes (found in the cytoplasm of eukaryotic cells).

Step 2: Detailed Explanation:
The 'S' unit refers to the Svedberg unit, a non-metric unit that measures the rate of sedimentation of particles during ultracentrifugation.
Sedimentation rate depends on both the mass and the physical shape of the molecule, which is why subunit values are not directly additive.
Eukaryotic cytoplasmic ribosomes (80S) dissociate into two distinct ribonucleoprotein subunits:
1. The small subunit (40S): This subunit contains a single 18S ribosomal RNA (rRNA) molecule complexed with approximately 33 proteins.
2. The large subunit (60S): This subunit consists of three distinct rRNA molecules (28S, 5.8S, and 5S) and about 46 ribosomal proteins.
When these subunits associate during the initiation of translation, they form the functional 80S ribosome.
In contrast, the 70S prokaryotic ribosome consists of a 30S small subunit and a 50S large subunit.

Step 3: Final Answer:
Thus, the subunits of the 80S ribosome are the 40S and 60S subunits.