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A ribosome is a biological molecule made of ribonucleic acid (RNA) and proteins (ribosomal proteins). The structure of a ribosome is complex, and it is responsible for making the millions of proteins that are needed by cells. Think of a ribosome as a small protein biosynthetic factory that translates the DNA genetic information into an amino acid sequence (the primary structure of proteins).
A ribosome may be located in many places within the cell. Some are in the cytosol (free ribosomes). Others are bound to cellular membranes. Membrane-bound ribosomes are responsible for the characteristic roughness of the endoplasmic reticulum when seen under a microscope.
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Structure of Ribosomes
A ribosome is made of two pieces (subunits). These two subunits are named according to their ability to sediment on a special gel (the Svedberg unit, a measure of the rate of sedimentation in centrifugation). The bigger the number given to the subunit the bigger the molecule.
A typical eukaryotic cell ribosome consists of two subunits named 60S (large subunit) and 40S (small). According to Alberts et al (2002) the 60S subunit is made of a 5S RNA (of 120 nucleotides), a 28S RNA (of 4700 nucleotides), a 5.8S subunit (of 160 nucleotides) and around 49 proteins. The 40S subunit, on the contrary, has a 1900 nucleotide (18S) RNA and approximately 33 proteins. A prokaryotic cell ribosome is a little smaller but it is made of two subunits too: a 50S and 30S subunit.
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Function of Ribosomes
The ribosome is responsible for manufacturing the proteins. In each living cell, the information contained in the DNA (located in the nucleus) is "transferred" to a messenger RNA (mRNA). The mRNA leaves the nucleus and travels to the endoplasmic reticulum (or the cytosol) where the two ribosome subunits assemble around it and start synthesizing proteins. This is done by a process called "translation," which is basically translating the mRNA information into an amino acid sequence. The actual process is quite complex, but in essence thanks to the ribosome the actual proteins (needed by the cell) are assembled.
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Is a Ribosome Similar to an Enzyme?
There is active discussion in academic and research circles regarding the nature of ribosomes. Since they have the ability to efficiently catalyze the assembly of proteins many think of ribosomes as enzymes. However, since by definition an enzyme is a protein and ribosomes contain RNA many have termed this new type of biocatalyst as a “ribozyme.”
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Alberts et al. (2002). Molecular Biology of the Cell, 4th ed. Garland Science.
Yusupov et al. (2001). Crystal Structure of the Ribosome. Science 292: 883-896