Despite protein biosynthesis being studied for decades, some major questions concerning this process are still to be addressed. We elucidate a close connection between proofreading of the emerging amino acid sequence during its normal, elongation factor-dependent ribosomal biosynthesis and the existence of the factor-free synthesis of a polypeptide chain on a ribosome. In this factor-free process, the biological role of proofreading is played by a process opposite to the factor-free attachment of Aa-tRNA to the ribosome, namely, the removal via the same pathway of that Aa-tRNA, which is not complementary to the mRNA codon exhibited by the ribosome.