Ribosomes
"Ribosomes are the cell's protein synthesis machinery"
Ribosomes are small complexes of RNA and protein that are sites of protein synthesis.
Ribosomes are among the most complex molecular assemblies
found in cells. Each ribosome is composed of two subunits (see figure below), and each subunit is composed of a combination of RNA,
called ribosomal RNA (rRNA), and several dozen different proteins.
The subunits join to form a functional ribosome only when they are
actively synthesizing proteins. This complicated process requires the
two other main forms of RNA: messenger RNA (mRNA), which carries coding information from DNA, and transfer RNA (tRNA),
which carries amino acids. Ribosomes use the information in mRNA
to direct the synthesis of a protein. This process is often regulated by
other small RNA molecules.
Ribosomes consist of a large subunit and a small subunit composed of rRNA and protein. The individual subunits are synthesized in the nucleus and then move
through the nuclear pores to the cytoplasm, where they assemble
to translate mRNA. Ribosomes serve as sites of protein synthesis.
Ribosomes are found either free in the cytoplasm or associated with internal membranes called endoplasmic reticulum(ER). Free ribosomes synthesize proteins that
are found in the cytoplasm, nuclear proteins, mitochondrial proteins, and proteins in other organelles not derived from the endomembrane system. Membrane-associated ribosomes synthesize
membrane proteins, proteins found in the endomembrane system,
and proteins destined for export from the cell.
Ribosomes can be thought of as “universal organelles,”
because they are found in all cell types from all three domains of
life. As we build a picture of the minimal essential functions for
cellular life, ribosomes will be on the short list. Life is protein-based, and ribosomes are the factories that make proteins.
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