RNA synthesis, carried out by RNA polymerase, has both similarities and differences compared to DNA replication, carried out by DNA polymerase. In both, the synthesis occurs by means of the nucleophilic addition-elimination upon a phosphate group, similar to addition-elimination upon carboxylic acid derivatives, made thermodynamically favorable by phosphate cleavage. In both, the direction of synthesis is 5' to 3'. However, unlike DNA polymerase, RNA polymerase carries out RNA synthesis without a primer and unlike DNA polymerase, RNA polymerase does not detect and excise mismatched nucleotides.

In eukaryotes, transcription factors interact with promoters, binding upstream and assisting in RNA polymerase activity. Eukaryotes also possess enhancer sequences. The binding of a glucocorticoid receptor complex to an enhancer sequence, for example, stimulates transcription of certain genes.