<P> Transcription is the first step of gene expression, in which a particular segment of DNA is copied into RNA (especially mRNA) by the enzyme RNA polymerase . Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language . During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand called a primary transcript . </P> <P> Transcription proceeds in the following general steps: </P>

When is messenger rna made using dna as the pattern