Transcription

The cell guards its DNA like a master copy in a library: it never leaves the nucleus. Instead, the cell makes a working photocopy, mRNA, that carries just one gene's instructions out to the ribosome. If something damages that copy, no problem; the original stays safe and the cell just makes another. This is also why transcription copies only a short stretch of DNA, one gene at a time, rather than the whole genome the way replication does.

RNA polymerase binds a promoter, a specific DNA sequence marking the start of a gene, and that binding tells it which of the two strands to read: it always reads the template strand 3' to 5', which means it builds the new mRNA strand 5' to 3', antiparallel to the template. Initiation is this binding step. In elongation, RNA polymerase slides along the template, adding RNA nucleotides one at a time by base-pairing: A, C, and G pair normally, but U pairs where the template has A, since RNA uses uracil instead of thymine. Termination happens when the polymerase reaches a terminator sequence and releases both the DNA and the finished mRNA strand.

It is tempting to think transcription is just replication with U instead of T, but the two processes differ in far more than that letter swap. RNA polymerase needs no primer, unlike DNA polymerase. The product is single-stranded, not a new double helix. And that mRNA is often temporary, edited and eventually broken down, not a permanent genetic record. This transient, disposable quality sets up the next step, where the mRNA is processed and translated into protein.