Ribosome

Ribosomes were first observed as small, dense particles or granules under the electron microscope by George Palade in 1953. Under closer electron-microscope study, each ribosome turns out to be built of two separate, rounded subunits (one smaller, one larger) that come together to form a single functional unit; the two subunits are held in that assembled state by magnesium ions (Mg2+), which are essential for the ribosome's structural cohesion. Ribosomes can arise inside a cell in three different ways: de novo formation, auto-replication, or a nucleolar origin. Unlike almost every other organelle discussed in this chapter, a ribosome is not bound by any membrane at all, and its actual job is to be the physical site of protein synthesis within the cell. There are two distinct sizes of ribosome, and their names come from the Svedberg unit (S) - a measure, named after the Swedish Nobel-laureate chemist Theodor Svedberg, of particle size based on how quickly a particle sediments in an ultracentrifuge. Prokaryotic cells (and, notably, the mitochondria and chloroplasts of many algae and higher plants) carry the smaller 70S ribosome, itself made up of a 30S small subunit and a 50S large subunit, together holding three distinct rRNA molecules (16S rRNA in the small subunit, 23S and 5S rRNA in the large subunit). Eukaryotic cytoplasmic ribosomes, in plants and animals, are the larger 80S type, built of a 40S small subunit and a 60S large subunit, together holding four rRNA molecules (18S rRNA in the small subunit, and 28S, 5.8S and 5S rRNA in the large subunit). By mass, a ribosome is roughly 60% RNA and 40% protein. During active protein synthesis, many individual ribosomes attach simultaneously to a single strand of messenger RNA, forming a structure called a polysome or polyribosome; the purpose of this arrangement is to …