Complex sentence: In contrast to producing viral mRNA for viral protein synthesis, when replicating the viral genome, the polymerase does not start and stop at the indicated cis-acting elements between each gene, instead it elongates the RNA along the entire length of the genome.
Split into simpler sentences: In contrast to producing viral mRNA for viral protein synthesis, when replicating the viral genome, the polymerase does not start and stop at the indicated cis-acting elements between each gene. Instead, the polymerase elongates the RNA along the entire length of the genome.