A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Transcription is the synthesis of an RNA molecule complementary to a DNA template, carried out by RNA polymerase. The enzyme binds to a promoter sequence upstream of the gene, unwinds the double helix, and synthesizes RNA in the 5'-to-3' direction using the template (antisense) strand. In prokaryotes, a single RNA polymerase handles all RNA types; eukaryotes use three specialized polymerases (RNA Pol I, II, III). The product is a primary transcript that in eukaryotes requires further processing before translation.
Map promoter elements (TATA box, -10/-35 boxes) and trace the polymerase through initiation, elongation, and termination. Compare prokaryotic and eukaryotic transcription side by side.
Transcription is the first step in converting the genetic information stored in DNA into a functional product. The central idea is straightforward: one strand of the DNA double helix is used as a template to synthesize a complementary RNA molecule. But the details of how this happens — and how it differs between prokaryotes and eukaryotes — reveal a great deal about how cells control which genes are expressed.
The process begins with initiation. RNA polymerase must recognize where to start. It does this by binding to a specific DNA sequence called a *promoter*, located upstream (in the 5' direction of the coding strand) from the gene. In prokaryotes, promoters have conserved sequences around positions −10 and −35 relative to the transcription start site. In eukaryotes, promoters are more complex and often include a TATA box, and RNA Pol II requires a set of *transcription factors* to assemble at the promoter before the polymerase can bind. This added complexity is a major mechanism for differential gene expression in eukaryotes.
Once bound, RNA polymerase unwinds a short stretch of the double helix and begins elongation: reading the template strand in the 3'→5' direction and synthesizing the RNA strand in the 5'→3' direction. The base-pairing rules are the same as in DNA replication — A pairs with U (not T, since RNA uses uracil), T pairs with A, C with G, G with C. A key point worth emphasizing: the RNA produced is *not* complementary to the coding strand — it is *identical* to it (with U replacing T). Students often confuse this because the polymerase physically reads the template strand, but the product mirrors the coding strand. This is why the coding strand is sometimes called the "sense strand."
Termination occurs when the polymerase reaches a terminator sequence. In prokaryotes, this can be a hairpin loop in the nascent RNA that causes the polymerase to stall and dissociate. In eukaryotes, termination is coupled to cleavage and polyadenylation of the transcript. After termination, eukaryotic pre-mRNA undergoes extensive *processing* before it can be translated: a 5' cap (a modified guanosine) and a poly-A tail are added for stability and nuclear export, and *introns* — non-coding intervening sequences — are spliced out by the spliceosome, leaving only the coding exons joined together.
The contrast between prokaryotic and eukaryotic transcription illustrates a broader principle: complexity in gene regulation scales with organismal complexity. Prokaryotes sacrifice regulatory sophistication for speed — they can translate mRNA while it is still being transcribed because there is no nuclear membrane separating the two processes. Eukaryotes pay a time and energy cost in RNA processing but gain multiple checkpoints at which gene expression can be regulated, enabling the cell-type-specific gene expression that underlies development.