Analogous problem to the secondary structure of proteins. Likely a bit simpler due to the strong tendency for complementary pairs to bind.
A DNA sequence that marks the start of a transcription area.
Converts DNA to RNA.
Makes RNA from RNA.
Used in Positive-strand RNA virus to replicate.
I don't think it's present outside viruses. Well regulated organisms just transcribe more DNA instead.
Sequence of genes under a single promoter. For an example, see E. Coli K-12 MG1655 operon thrLABC.
A single operon may produce multiple different transcription units depending on certain conditions, see: operon vs transcription unit.
A sequence of mRNA that can actually be transcribed.
Multiple different transcription units can be produced by a single operon, see: operon vs transcription unit.
That single operon can produce two different mRNA transcription units:
The reason for this appears to be that there is a rho-independent termination region after thrL. But then under certain conditions, that must get innactivated, and then the thrLABC is produced instead.
The most important ones are:

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