RNA polymerase moves stepwise along the DNA, unwinding the DNA helix in front of it to expose a new region of the template strand for complementary base-pairing. As it progresses, the polymerase adds ribonucleotides one-by- one to the RNA chain, using an exposed DNA strand as a template. In this way, the growing RNA chain is elongated by one nucleotide at a time in the 5ʹ-to-3ʹ direction.

The resulting RNA transcript is thus single-stranded and complementary to the template strand. The incoming ribonucleoside triphosphates (ATP, CTP, UTP, and GTP) provide the energy needed to drive the reaction forward, analogous to the process of DNA synthesis.

As the polymerase moves along the DNA template, it displaces the newly formed RNA, allowing the two strands of DNA behind the polymerase to rewind.

A short region of hybrid DNA/RNA helix (approximately nine nucleotides in length) therefore forms only transiently, causing a “window” of DNA/RNA helix to move along the DNA with the polymerase.