Is this about the question where I removed the "dna" tag? :-) I didn't realize the DNA connection until I started reading your answer. Feel free to add it back.
I doubt tagging answers would make sense. I think it's better to spin off discussions that take a different turn.
With a different question, you can change the headline. I think it's confusing if the main subject of a question has nothing to do with a topic to have it tagged for that topic. People don't know why they'd want to click over to it.