transcription
/ tran-SKRIP-shun /
Imagine the master cookbook of the cell — the DNA — is so precious that it never leaves the kitchen. When you want to actually cook one recipe, you don't drag the whole heavy book to the counter; you copy out just that one page onto a slip of paper and work from the slip. Transcription is the cell making that working copy. It reads one gene's stretch of DNA and writes out a matching strand of RNA, leaving the original DNA safely intact.
More precisely, transcription is the synthesis of an RNA molecule from a DNA template, carried out by an enzyme called RNA polymerase. The polymerase opens a short stretch of the double helix, latches onto one of the two strands (the template strand), and reads it base by base. For every base it reads, it adds the matching RNA building block to a growing chain: where the DNA template says C the RNA gets G, where it says A the RNA gets U (uracil, RNA's stand-in for the thymine used in DNA), and so on. The result is a single-stranded RNA copy whose sequence mirrors the gene. The job proceeds in three acts — initiation (getting started at the right place), elongation (steadily copying), and termination (knowing where to stop).
Transcription is the first step in turning a gene into something the cell can use — the opening move of gene expression, captured in the central dogma's arrow DNA -> RNA -> protein. It matters enormously because it is the main place the cell decides which genes to switch on and how loudly: nearly every cell in your body carries the same DNA, yet a neuron and a skin cell differ because they transcribe different genes. This entry covers the molecular act of copying DNA into RNA; what happens to that RNA afterward, and the logic of when a gene is switched on, are separate stories.
A gene's template strand reads 3'-TACGGATC-5'. RNA polymerase transcribes it into the RNA 5'-AUGCCUAG-3' — note U appears wherever the template had A, and the RNA sequence matches the other (coding) DNA strand except T becomes U.
Transcription copies one DNA strand into a complementary RNA, with uracil in place of thymine.
Transcription does not consume or destroy the DNA — the template is read and then re-zipped, so the same gene can be transcribed thousands of times. Replication copies the whole genome once per division; transcription copies single genes on demand, over and over.