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Translation: Ribosomes and Initiation
Ribosomes are the machinery of biological cells that are responsible for synthesizing proteins via a process known as translation. Ribosomes themselves consist of two subunits, a large and a small subunit. In eukaryotic cells, the small subunit is the 40S while the large subunit is the 60S subunit. In prokaryotic cells, the small subunit is the 30S and the large subunit is the 50S. The letter "S" stands for the Svedberg unit, which is the unit that describes the rate at which the particle sediments. The higher the value is, the higher the rate at which it travels down the test tube when the test tube is placed into the centrifuge. The small and the large subunits only come together to form the ribosome during the process of translation. Translation can be broken down into three stages - initiation, elongation and termination. During initiation, the mRNA molecule seeks out the small ribosomal subunit with the help of special proteins known as initiation factors. When the small subunit binds to the mRNA molecule, it slides along the mRNA until it reaches a special three-nucleotide sequence known as the start codon. Once the small subunit finds the start codon, it signals a tRNA molecule to locate and bring the methionine amino acid. Once the aminoacyl tRNA complex (the tRNA along with the amino acid) attaches to the start codon, this signals the large subunit to attach to the small subunit, thereby forming the ribosome complex. This concludes the process of initiation.
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