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A transcription bubble is a molecular structure that occurs during the transcription or replication of DNA when DNA helicase and DNA topoisomerase "unzip" the DNA double strand. DNA polymerase or RNA polymerase (Main Articles: DNA replication, transcription) may then bind to the exposed DNA and begin synthesizing a new strand of DNA or RNA. As DNA and/or RNA polymerase progresses down the DNA strand in the 3' to 5' direction, more of the DNA double strand is unwound, creating a replication or transcription bubble in the process that may be seen with specialized staining techniques and microscopy.

See also: Okazaki fragment, primase, transcription factors, replication initiation complex, transcription initiation