The process of elongation is the synthesis of a copy of the DNA into messenger RNA. RNA Pol II matches complementary RNA nucleotides to the template DNA by Watson-Crick base pairing. These RNA nucleotides are ligated, resulting in a strand of messenger RNA.
Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases on a single DNA template and multiple rounds of transcription (amplification of particular mRNA), so many mRNA molecules can be rapidly produced from a single copy of a gene.
Elongation also involves a proofreading mechanism that can replace incorrectly incorporated bases. In eukaryotes, this may correspond with short pauses during transcription that allow appropriate RNA editing factors to bind. These pauses may be intrinsic to the RNA polymerase or due to chromatin structure.