DNA & mRNA During Transcription
Transcription occurs in a special structure known as transcription bubble. Inside the bubble are present the mRNA, template DNA being transcribed and the RNA Polymerase. Upstream of bubble is the DNA already transcribed and downstream is the DNA to be transcribed. There is not enough space in cell to have completely unfolded DNA for transcription, so transcribed DNA has to be re-annealed as soon as it is transcribed, giving a bubble-like appearance to the RNA polymerase-bound DNA. Hence it is called a transcription bubble. See this image for illustration:


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The mRNA formed quickly leaves the bubble and hangs behind like a thread coming from RNA polymerase.


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