70S ribosomes are ribosomes found in prokaryotic cells and in the chloroplasts and mitochondria of eukaryotic cells.
Characteristics of 70S Ribosomes
- Location: Present in the cytoplasm of prokaryotic cells (bacteria and archaea) and within the mitochondria and chloroplasts of eukaryotic cells.
- Size: Smaller than the 80S ribosomes found in the cytoplasm of eukaryotic cells.
- Subunits: Composed of two subunits:
- 50S subunit: The larger subunit.
- 30S subunit: The smaller subunit.
It's important to note that the 'S' in 70S, 50S, and 30S stands for Svedberg units, which are a measure of sedimentation rate during centrifugation and not directly additive (hence 50S + 30S = 70S, not 80S). Sedimentation rate depends on both mass and shape.
Function of 70S Ribosomes
70S ribosomes are essential for protein synthesis (translation) in the organisms and organelles where they are found. They bind to messenger RNA (mRNA) and transfer RNA (tRNA) to assemble amino acids into polypeptide chains, which then fold into functional proteins.
Significance
The presence of 70S ribosomes in mitochondria and chloroplasts provides evidence for the endosymbiotic theory, which proposes that these organelles were once free-living prokaryotic cells that were engulfed by eukaryotic cells. The difference between 70S and 80S ribosomes is also a target for some antibiotics, which can selectively inhibit protein synthesis in bacteria without harming eukaryotic cells.
