What is formed during Rho-independent termination in RNA?

During Rho-independent termination in RNA synthesis, a specific structure known as a hairpin loop is formed. This process involves the formation of a stable secondary structure in the RNA molecule that is transcribed. As RNA polymerase moves along the DNA template to synthesize RNA, certain sequences within the RNA can lead to the formation of this hairpin loop, which typically consists of inverted repeat sequences.

When the hairpin loop forms, it creates a pause in the transcription process due to its stability. This pause is critical because it provides the necessary time for RNA polymerase to disassociate from the DNA, ultimately leading to the termination of transcription. The formation of the hairpin loop is a key feature of Rho-independent termination as it effectively signals the end of RNA synthesis without the need for a protein factor like Rho.

In contrast, the other options do not describe the structure formed during Rho-independent termination. A transcription bubble refers to the unwinding of DNA during transcription, a double-stranded complex would imply pairs of double-stranded nucleic acids, and an open reading frame is a segment of RNA that can be translated into a protein, which is unrelated to the termination mechanism itself. The hairpin loop thus stands out as the correct answer