5.7: Ribosomes
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Ribosomes are composed of rRNA and protein and consist of 2 subunits. In eukaryotic cells, the subunits have densities of 60S and 40S ("S" refers to a unit of density called the Svedberg unit) and are composed of longer rRNA molecules and more proteins than the 50S and 30S subunits found in prokaryotic ribosomes. When the two ribosomal subunits join together during translation, they form a complete ribosome having a density of 80S.
The ribosomes are both attached to the endoplasmic reticulum and free in the cytoplasm. They serve as a workbench for protein synthesis, that is, they receive and translate genetic instructions for the formation of specific proteins or polypeptides.