Helicase unwinds the parental duplex at the rate set above, exposing single-stranded template on both sides of the fork. DNA polymerase can only extend a strand 5'→3', so the two exposed templates are copied differently:
- Leading strand — polymerase runs continuously in the same direction the fork opens, so one new strand is made as a single unbroken chain. If its speed slider is set below the helicase rate, a gap of exposed single-stranded template (ssDNA) grows between the polymerase and the fork — visible as a bare stretch in the diagram.
- Lagging strand — its template runs the wrong way for continuous synthesis, so primase lays a short RNA primer (orange) every "Okazaki fragment length" bases behind the fork, and polymerase extends each primer backward until it bumps into the previous fragment. DNA ligase then seals the nick, joining the fragments into one continuous strand (shown as the fragment turning solid).
Every newly added nucleotide is chosen by real Watson–Crick pairing against its template base: A↔T, G↔C — never arbitrary, and colored consistently (A cyan, T red, G green, C yellow) on both new strands.