Model Answer
0 min readIntroduction
Ribosomes are complex molecular machines responsible for protein synthesis (translation) in all living organisms. They are not membrane-bound organelles and are found in both prokaryotic and eukaryotic cells. Ribosomes are classified into two main types based on their sedimentation coefficient during ultracentrifugation: 70S ribosomes found in prokaryotes, mitochondria, and chloroplasts, and 30S ribosomes which are a subunit of the 70S ribosome, and 80S ribosomes found in eukaryotic cytoplasm. These differences in structure and composition are crucial for understanding their function and are exploited by certain antibiotics to selectively inhibit bacterial protein synthesis. This answer will comprehensively compare the structure and function of 30S and 70S ribosomes.
Structural Comparison of 30S and 70S Ribosomes
The primary structural difference between 30S and 70S ribosomes lies in their size, composition, and the types of ribosomal RNA (rRNA) and proteins they contain. The 'S' value (Svedberg unit) is a measure of sedimentation rate and is related to size and shape.
| Feature | 30S Ribosome (Prokaryotic Subunit) | 70S Ribosome (Prokaryotic Complete Ribosome) |
|---|---|---|
| Sedimentation Coefficient | 30S | 70S |
| Molecular Weight (Daltons) | ~900,000 | ~2,700,000 |
| rRNA Molecules | 16S rRNA | 23S rRNA & 5S rRNA |
| Number of Proteins | ~21 proteins | ~55 proteins |
| Composition | 16S rRNA + ~21 ribosomal proteins | 30S subunit (16S rRNA + ~21 proteins) + 50S subunit (23S rRNA, 5S rRNA + ~34 proteins) |
| Location | Part of the 70S ribosome in prokaryotes | Prokaryotic cytoplasm, mitochondria, and chloroplasts |
Functional Roles and Differences
Both 30S and 70S ribosomes perform the essential function of protein synthesis, but they differ in their specific roles and sensitivity to inhibitors.
30S Ribosomal Function
- Initiation of Translation: The 30S subunit plays a crucial role in recognizing the Shine-Dalgarno sequence (in prokaryotes) or the Kozak sequence (in eukaryotes) on mRNA, which helps position the mRNA correctly for translation initiation.
- tRNA Binding: It binds to the initiator tRNA carrying formylmethionine (in prokaryotes) or methionine (in eukaryotes) and ensures correct codon-anticodon pairing.
- Decoding mRNA: The 30S subunit is primarily responsible for accurately decoding the mRNA sequence during translation.
- Antibiotic Targets: The 30S subunit is a common target for several antibiotics, including streptomycin, tetracycline, and kanamycin. These antibiotics interfere with different stages of translation, such as initiation, elongation, or proofreading.
70S Ribosomal Function
- Overall Protein Synthesis: The 70S ribosome, as a complete unit, carries out the entire process of protein synthesis – initiation, elongation, and termination.
- Peptide Bond Formation: The 50S subunit (part of the 70S ribosome) contains the peptidyl transferase center, which catalyzes the formation of peptide bonds between amino acids.
- Translocation: The 70S ribosome translocates along the mRNA, moving from one codon to the next.
- Ribosome Recycling: After translation is complete, the 70S ribosome dissociates into its subunits for recycling.
- Antibiotic Targets: The 70S ribosome is also a target for antibiotics like chloramphenicol and erythromycin, which primarily affect the 50S subunit.
Differences in Antibiotic Sensitivity
The structural differences between 30S and 70S ribosomes explain their differential sensitivity to antibiotics. Eukaryotic 80S ribosomes are less susceptible to antibiotics that target 70S ribosomes, providing a degree of selective toxicity. For example, tetracycline binds to the 30S subunit and prevents tRNA attachment, inhibiting bacterial protein synthesis without significantly affecting eukaryotic cells.
Evolutionary Significance
The presence of 70S ribosomes in mitochondria and chloroplasts supports the endosymbiotic theory, which proposes that these organelles originated from ancient bacteria. The similarity in ribosomal structure between these organelles and bacteria provides strong evidence for their bacterial ancestry.
Conclusion
In conclusion, 30S and 70S ribosomes, while both essential for protein synthesis, exhibit significant structural and functional differences. The 30S subunit is crucial for initiation and decoding, while the 70S ribosome represents the complete functional unit. These differences are exploited by antibiotics to selectively target bacterial protein synthesis and are also reflected in the evolutionary history of organelles like mitochondria and chloroplasts. Understanding these distinctions is fundamental to comprehending the intricacies of gene expression and developing effective antimicrobial strategies.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.