The nitrogen base that is found during transcription but not replication is uracil (U).

Here's why:

* Transcription is the process of copying DNA into RNA. RNA uses uracil (U) instead of thymine (T).

* Replication is the process of copying DNA into new DNA molecules. DNA uses thymine (T).

So, during transcription, uracil is used to pair with adenine (A) in the RNA strand, while thymine is used to pair with adenine in the DNA strand during replication.