Coronavirus genomes also encode a protein called RNA-dependent RNA polymerase (RdRp), which allows the viral genome to be transcribed into new RNA copies using the host cell's machinery. The RdRp is the first protein to be made; once the gene encoding the RdRp is translated, translation is stopped by a stop codon. This is known as a nested transcript. When the mRNA transcript only encodes one gene, it is monocistronic. Coronavirus non-structural proteins provide extra fidelity to replication, because they confer a proofreading function, which is lacking in RNA-dependent RNA polymerase enzymes alone.

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