70S Ribosomes

Ribosomes are the sites of protein synthesis (translation). They are made of ribosomal RNA (rRNA) and protein, and each ribosome has two subunits, one large and one small. Ribosomes are not surrounded by a membrane.Prokaryotic cells have 70S ribosomes, made of a large 50S subunit and a small 30S subunit. The ribosomes in the cytoplasm of eukaryotic cells are larger, 80S, made of 60S and 40S subunits. In bacteria the ribosomes are free in the cytoplasm, because there is no endoplasmic reticulum.The letter S stands for the Svedberg unit, a measure of how fast a particle sediments when it is spun in an ultracentrifuge. It depends on size and shape, so S values are not additive: a 50S and a 30S subunit together make a 70S ribosome, not an 80S one.70S ribosomes are also found inside mitochondria and chloroplasts. This is one piece of evidence for the endosymbiotic theory, which proposes that these organelles came from free-living prokaryotes.The difference between 70S and 80S ribosomes matters in medicine. Some antibiotics bind to the 30S or 50S subunit of bacterial ribosomes and stop translation, so the bacterium cannot make proteins. Because they do not bind as well to 80S ribosomes, these drugs harm the bacterium far more than the human host cells. This is an example of selective toxicity.