primase and the RNA primer
/ PRY-mayss and the R-N-A PRY-mer /
Imagine trying to start sewing without first tying a knot or making one anchoring stitch — your thread would just pull straight through the cloth with nothing to build on. The main DNA-copying enzyme has the same problem: it can add new building blocks only onto an existing piece, never to bare ground. It needs a starting handhold. Primase is the enzyme that lays down that first little starter piece, and the starter piece it makes is called an RNA primer.
Primase reads the single-stranded DNA template and builds a short stretch — usually just a handful to a dozen or so units long — of RNA, a close chemical cousin of DNA, complementary to the template. That short RNA piece provides a free end onto which DNA polymerase can begin adding DNA building blocks. In other words, the primer is a temporary toehold: RNA is used rather than DNA precisely because primase, unlike DNA polymerase, can start a strand from nothing.
Primers matter because every new stretch of DNA must begin on one. On the smoothly copied leading strand a single primer near the start is usually enough, but on the lagging strand, which is built in many short pieces, a fresh primer is needed to begin each piece — so primase fires over and over. Crucially, RNA primers are only scaffolding: they are later removed and replaced with proper DNA, so the finished product contains no leftover RNA. A common misconception is that the primer becomes part of the final DNA; in fact it is always erased before copying is complete.
On the lagging strand, primase keeps dropping fresh RNA primers like stepping stones along the template. DNA polymerase starts from each primer and runs forward until it bumps into the next stone — producing the chain of short fragments that later get stitched together.
Each new DNA fragment must start from an RNA primer made by primase.
RNA primers are temporary scaffolding — they are later removed and replaced by DNA, so no RNA remains in the finished strand.