Codons are only meaningful for translation, since that is when the nucleo-based DNA/RNA sequence information is translated into a protein sequence based on the triplet code. Before that step codons don't really have 'meaning'.
During transcription the start & stop codons are already present, but they are ignored by the RNA polymerase, because it works on a single base at a time.
The equivalent 'start' and 'stop' signs for transcription are promoters) and poly-adenylation-sites (at least for eukaryotic mRNA). Both are recognised by additional proteins, that help regulate transcription:
* transcription factors bind the promoters on the DNA and recruit RNA polymerase to that site so that transcription can start
* poly-adenylation-site are specific sequences that lead to the addition of the poly-A tail to the nascent mRNA. This process also leads to termaination of transcription, however the exact mechanism isn't quite clear.