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What Are Ribosomes?
The ribosomes in eukaryotes are small, round organelles that are about 20 nanometers in diameter. It is so small that it is only visible with an electron microscope. Ribosomes are found in the cytoplasm of a cell. They are either free floating or bound to the endoplasmic reticulum. Groups of ribosomes stuck together are called polysomes. Most cells contain thousands of ribosomes, while a few specialized cells contain millions. Brain cells and the cells of the pancreas contain large concentrations of ribosomes since they produce a large amount of protein.
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The Structure of Ribosomes
Although ribosomes are called organelles, they don't have a membrane that holds the contents of the ribosome in one cohesive structure. Ribosomes are composed of ribosomal RNA (rRNA) and protein. Sixty percent of the structure is rRNA and 40 percent is protein. There are two subunits: a large subunit and a small subunit. They are labeled as 60s and 40s after the Svedberg values.
The subunits of ribosomes are produced in the nucleolus, which is located in the nucleus of the cell. Ribosomal proteins enter the nucleolus and combine with four rRNA strands to produce the two subunits. The subunits leave the nucleus through the nuclear pores and combine in the cytoplasm.
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Role in Protein Synthesis
What do ribosomes do? Ribosomes assemble a chain of amino acids to form a polypeptide, which eventually becomes a protein. Depending on the location of the ribosome, different types of proteins are produced. Generally, ribosomes that are bound to the endoplasmic reticulum construct proteins that are transported out of the cell including cell surface receptors. The free ribosomes produce proteins that are used internally by the cell. Some of the proteins produced by free ribosomes include the enzymes required for metabolizing glucose.
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mRNA and tRNA
What do ribosomes do to build a protein? Ribosomes interact with two types of RNA molecules to build a polypeptide. Messenger RNA (mRNA) contains genetic information about the structure of a protein. Transfer RNA (tRNA) transports individual amino acids to the ribosome. As the ribosome subunits come in contact with mRNA, tRNA brings an amino acid that corresponds with the genetic code. tRNA is bound to the ribosome at three sites: the aminoacyl binding site, peptidyl binding site and the exit binding site. tRNA holds the growing polypeptide, which can extend by two amino acids every second.
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After the polypeptide is constructed, it is transported from the ribosome to the Golgi apparatus. At this point, the ribosome may separate into two subunits since it isn't in the process of building a protein. In the Golgi apparatus, the polypeptide is processed. Additional molecules may be added or molecules may be removed. The protein is released and transported out of the cell or kept within the cell.
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1. Davidson, Michael W. "Ribosomes." Molecular Expressions Cell Biology Florida State University. http://micro.magnet.fsu.edu/cells/ribosomes/ribosomes.html
2. Kimball, John W. "Ribosomes." Kimball's Biology Pages. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/Ribosomes.html