In order to synthesise an RNA strand, transcription must start and stop at the appropriate places. How is this achieved?

The process of starting and stopping transcription at the appropriate places is achieved through specific sequences in the DNA that signal the start and stop of transcription. Here are the steps:

1. Initiation: Transcription begins at a specific sequence on the DNA called the promoter. The promoter is recognized and bound by a complex of proteins called transcription factors. These transcription factors recruit RNA polymerase, the enzyme that synthesizes the RNA strand, to the promoter.

2. Elongation: Once RNA polymerase is bound to the promoter, it unwinds the DNA and begins synthesizing the RNA strand. It does this by adding RNA nucleotides that are complementary to the DNA strand.

3. Termination: Transcription stops when RNA polymerase encounters a specific sequence on the DNA called the terminator. Upon reaching the terminator, RNA polymerase releases the RNA strand and detaches from the DNA.

4. Post-transcriptional modifications: After transcription, the RNA strand undergoes several modifications before it is ready to be translated into a protein. These include the addition of a 5' cap and a poly-A tail, as well as splicing to remove introns (non-coding regions).

So, the start and stop of transcription are controlled by specific sequences in the DNA (the promoter and terminator) and the action of various proteins and enzymes.