Translation of an mRNA message into its polypeptide product on the ribosome is a polymerization reaction, and can be divided into three phases: initiation, elongation, and termination. Initiation requires the assembly of the translational machinery from its individual components to form a complex that is primed for peptide bond formation. Formation of this initiation complex is usually the rate-limiting step in protein synthesis. Elongation is the sequential joining of amino acids by peptide bonds as dictated by the codons of the mRNA. Translation proceeds along the mRNA in the 5´ to 3´ direction, and the resulting polypeptide chain is synthesized in the amino-terminal to carboxy-terminal direction (Nto C-terminal). Termination occurs when a stop codon is reached in the message and the polypeptide is released from the ribosome, which is then recycled for translation of another protein. Each step has associated protein factors that facilitate substrate transport and ribosomal function.
The ribosome is a ribonucleoprotein particle, that is, it contains both RNA and protein components. The three RNA molecules (in prokaryotes, four in eukaryotes) and approximately 50 proteins that make up a ribosome are divided into two unequal subunits. Ribosomes and their subunits are characterized by their rate of movement (sedimentation) in intense centrifugal fields that greatly increase the force of gravity. Their sedimentation coefficients (S) are determined by size and shape. Bacterial ribosomes are 70S particles made up of 30S and 50S subunits. The smaller subunit (30S) contains the decoding site, where the mRNA–tRNA base-pairing interaction occurs. Peptide bond formation takes place on the larger ribosomal subunit (50S), at the peptidyl transferase center. When assembled, the subunits together form three binding sites for tRNA molecules. The A-site preferentially binds aminoacyl–tRNA, and is therefore where each incoming aminoacyl–tRNA complex is delivered for decoding. The nascent polypeptide chain is attached to the tRNA molecule in the P-site (for peptidyl–tRNA). Finally the E-site is the location of a deacylated tRNA molecule prior to its exit from the ribosome (E is for exit).
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