= Transcription elongation complex
A <transcription elongation complex> is the working assembly of <RNA polymerase>, its <DNA> template and the growing <RNA> during transcript extension. The <RNA> $3^{\prime}$ end is positioned for addition of the next <nucleotide>; a short <DNA-RNA hybrid> helps organize the nucleic acids inside the <enzyme>. Additional <proteins> can associate with this core assembly.
A 2001 <RNA polymerase II> crystal structure at $3.3\,\text{Å}$ resolution showed entering duplex <DNA>, unwinding before the <active site>, and a nine-<base pair> <DNA-RNA hybrid>. Compared with free <RNA polymerase II>, the <protein> clamp closed around the nucleic acids. These observations connected a structurally identifiable assembly with ongoing <transcription (biology)>.<Structural Basis of Transcription: An RNA Polymerase II Elongation Complex at 3.3 Å Resolution by Averell L. Gnatt and others>[Abstract: duplex entry, hybrid, RNA end and clamp changes]{ref}
For a sequence example, a template segment read $3^{\prime}$-TAC-$5^{\prime}$ specifies the complementary <RNA> segment $5^{\prime}$-AUG-$3^{\prime}$. Each addition extends the <RNA> while the complex advances relative to the template. This illustrates information flow; the three-letter example does not specify the complete set of contacts that stabilizes a real <transcription elongation complex>.
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