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.

Articles by others on the same topic (1)

Transcription is the biological process through which the information encoded in a gene's DNA is copied into messenger RNA (mRNA). This is the first step in gene expression, leading to the synthesis of proteins. Here's a brief overview of the transcription process: 1. **Initiation**: The transcription process begins when the enzyme RNA polymerase binds to a specific region of the DNA called the promoter, which is located near the start of a gene.