Still, the error rates are by far larger than those of replication . A multitude of mRNA and protein copies per gene are produced during transcription. Each cell can survive if some of them are faulty. But just one molecule is doubled during replication. The many steps of gene expression allow controls at different levels.
Differentiation is achieved by differential gene expression.
Changes may occur abruptly or fluidly. The binding of a mRNA to a ribosome, for example, is dependent on its recognition sequence. But not all types of mRNA are equipped with the same sequence. Consequently are those with recognition sequences with the highest affinity bound most. At low affinity is the probability that a mRNA is bound by a ribosome low, too, and as a consequence are only few protein molecules produced.