= Transcription
{disambiguate=biology}
{wiki}
= Transcription
{synonym}
<Transcription> is synthesis of <RNA> using one <DNA> strand as a template by an <RNA polymerase>. During elongation the <RNA polymerase> reads that template $3'\to5'$ while extending <RNA> $5'\to3'$. Only the selected region is transcribed; other regions need not be active, and the template strand can differ between <genes>. See https://www.ncbi.nlm.nih.gov/books/NBK26887/[Alberts et al.: RNA polymerase and selection of transcribed DNA regions].
For the displayed example, align these three <nucleotide sequences> in the same left-to-right positions:
$$
\begin{aligned}
\text{coding DNA:}\quad&5'\text{-}\mathrm{ATGGCTTAA}\text{-}3',\\
\text{template DNA:}\quad&3'\text{-}\mathrm{TACCGAATT}\text{-}5',\\
\text{RNA product:}\quad&5'\text{-}\mathrm{AUGGCUUAA}\text{-}3'.
\end{aligned}
$$
The first template triplet TAC selects AUG: T pairs with A, A with U, and C with G. The next CGA selects GCU, and ATT selects UAA. The <coding strand> therefore matches the <RNA> lettering except for T versus U, whereas the template is complementary. Reading that template in its own $5'\to3'$ direction gives $5'\text{-}\mathrm{TTAAGCCAT}\text{-}3'$; orientation cannot be discarded. The <gene expression> diagram keeps all three rows aligned. See https://openstax.org/books/biology-2e/pages/15-2-prokaryotic-transcription[Clark, Douglas and Choi, Biology 2e: template and coding strands in transcription].
This correspondence makes <nucleotide sequences> interpretable, but replacing T by U is valid only after identifying the <coding strand>. Moreover, <RNA> may be processed after <transcription>, so the initial product and mature <messenger RNA> need not have identical uninterrupted <nucleotide sequences>. <Transcription> also makes functional <RNA> that will not undergo <biological translation>. See https://www.ncbi.nlm.nih.gov/books/NBK26887/[Alberts et al.: RNA processing and RNA as the final gene product].
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