Ribosomes
are the sites where the cell assembles proteins according to genetic
instructions. A bacterial cell may have a few thousand ribosomes, although
a human cell has a few million. Cells that have high rates of protein synthesis
have a particularly great number of ribosomes. Cells active in protein synthesis
also have prominent nucleoli, which make the ribosomes.
Ribosomes function in two cytoplasmic areas. Free ribosomes are spread throughout the cytosol, while bound ribosomes are attached to the outside of a membranous network, endoplasmic reticulum. Most of the proteins that are made by free ribosomes will function inside the cytosol. The proteins produced by bound ribosomes usually exported from the cell.
Each ribosome is built from two subunits, each having its own mix of ribosomal RNA and proteins. Ribosomes are built with RNA from the nucleolus and are made in the nucleolus istself. These subunits join together to form a functional ribosome only when they attach to a messenger RNA molecule. The ribosomes present in eukaryotic cells are slightly larger than those found in prokaryotic cells.
Ribosomes function in protein synthesis. As they move along messenger RNA, amino acids are joined in an order originally dictated by DNA. Several ribosomes can be moving along the same messenger RNA at once and the entire complex is called a polysome.
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