Ribosomes can float free in cytosol or attach to endoplasmic reticulum. Polyribosomes are clusters of several ribosomes synthesizing the same protein. Nearly all polypeptides have signal sequences that target them for final location in cell. In translation, mRNA codons base-pair with tRNA anticodons carrying specific amino acids. Codon order determines order of tRNA molecules and sequence of amino acids in polypeptides.
Protein synthesis involves chain initiation, chain elongation, chain termination.
Initiation factor proteins are required to bring necessary translation components together. The polypeptide is transferred and attached by a peptide bond to the newly arrived amino acid. Reaction is catalyzed by a ribozyme, which is part of the larger subunit. The tRNA molecule in the P site leaves. The complete cycle is rapidly repeated, about 15 times per second in Escherichia coli .