Model Answer
0 min readIntroduction
Ribosomes are complex molecular machines found within all living cells, responsible for protein synthesis – a process crucial for life. Often referred to as the ‘protein factories’ of the cell, they translate genetic code from messenger RNA (mRNA) into polypeptide chains. Discovered by George Palade and Christian de Duve in the 1950s, ribosomes are not membrane-bound organelles, distinguishing them from structures like mitochondria or the endoplasmic reticulum. Understanding their intricate structure and chemical composition is fundamental to comprehending the mechanisms of gene expression and cellular function. This answer will detail the structure and chemical composition of ribosomes, highlighting differences between prokaryotic and eukaryotic systems.
Ribosome Structure
Ribosomes are composed of two major subunits: a large subunit and a small subunit. Each subunit is made up of ribosomal RNA (rRNA) molecules and ribosomal proteins. The size of ribosomes is measured in Svedberg units (S), which are a measure of sedimentation rate during centrifugation. Prokaryotic ribosomes are 70S, while eukaryotic ribosomes are 80S. This difference in size reflects variations in their composition.
Prokaryotic Ribosomes (70S)
The 70S ribosome consists of a 50S large subunit and a 30S small subunit.
- 30S Subunit: Contains a 16S rRNA molecule and approximately 21 proteins. The 16S rRNA plays a crucial role in mRNA binding and codon-anticodon recognition.
- 50S Subunit: Contains 23S rRNA, 5S rRNA, and approximately 31 proteins. The 23S rRNA possesses peptidyl transferase activity, catalyzing the formation of peptide bonds between amino acids.
Eukaryotic Ribosomes (80S)
The 80S ribosome consists of a 60S large subunit and a 40S small subunit.
- 40S Subunit: Contains an 18S rRNA molecule and approximately 33 proteins. Similar to the 16S rRNA, the 18S rRNA is involved in mRNA binding.
- 60S Subunit: Contains 28S rRNA, 5.8S rRNA, 5S rRNA, and approximately 49 proteins. The 28S rRNA also exhibits peptidyl transferase activity.
Chemical Composition
The chemical composition of ribosomes revolves around rRNA and ribosomal proteins.
Ribosomal RNA (rRNA)
rRNA molecules are not merely structural components; they play catalytic and functional roles in protein synthesis. Different rRNA molecules are present in each subunit, as described above. These rRNA molecules are transcribed from DNA in the nucleolus (in eukaryotes) and are subsequently assembled with ribosomal proteins.
Ribosomal Proteins
Ribosomal proteins are responsible for stabilizing the ribosome structure and contributing to its functional activity. They are encoded by genes located outside the nucleolus. These proteins bind to the rRNA molecules, forming a ribonucleoprotein complex. The specific proteins present vary between prokaryotic and eukaryotic ribosomes.
Comparative Table: Prokaryotic vs. Eukaryotic Ribosomes
| Feature | Prokaryotic (70S) | Eukaryotic (80S) |
|---|---|---|
| Overall Size | 70S | 80S |
| Large Subunit | 50S | 60S |
| Small Subunit | 30S | 40S |
| Large Subunit rRNA | 23S, 5S | 28S, 5.8S, 5S |
| Small Subunit rRNA | 16S | 18S |
| Number of Proteins | ~52 | ~80 |
| Location | Cytoplasm | Cytoplasm, Rough Endoplasmic Reticulum, Mitochondria, Chloroplasts |
The differences in ribosome structure and composition are exploited by certain antibiotics. For example, streptomycin and tetracycline specifically target prokaryotic ribosomes, inhibiting bacterial protein synthesis without significantly affecting eukaryotic cells. This selectivity makes them effective antibacterial agents.
Conclusion
In conclusion, ribosomes are essential cellular components responsible for protein synthesis. Their structure, comprising large and small subunits composed of rRNA and proteins, varies between prokaryotes and eukaryotes. These structural differences are reflected in their sedimentation rates (70S vs. 80S) and the specific rRNA molecules and proteins present in each subunit. Understanding the intricate details of ribosome structure and composition is crucial for comprehending the fundamental processes of gene expression and for developing targeted therapeutic interventions. Further research continues to unravel the complexities of ribosome function and its role in cellular health and disease.
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.