DNA to RNA Transcription Tool

The page carries a single input, labelled as the coding strand and pre-filled with ATGCGTACGTAC. The script does three things in order: it uppercases the text, swaps every T for a U, and deletes any character that is not A, U, G or C. The sample sequence returns AUGCGUACGUAC, and the box refreshes on every keystroke. This is string substitution, not a model of RNA polymerase. No promoter is recognised, no introns are spliced out, no 7-methylguanosine cap is added at the 5' end and there is no poly-A tail. What you get is the primary transcript sequence, in whatever reading frame you pasted.

Which strand you paste is where people trip. Messenger RNA carries the same sequence as the coding (sense) strand, with U standing in for T. The template strand is the one polymerase reads 3' to 5', and it is complementary and antiparallel to the transcript, so swapping T for U on the template hands you the reverse complement of the real mRNA. Work it through: coding strand ATGCGTACGTAC becomes AUGCGUACGUAC, which opens with the AUG start codon. Its template, written 5' to 3', is GTACGTACGCAT, and the same naive swap would give GUACGUACGCAU, coding for nothing you asked for. The field label, the page description and the code all ask for the coding strand. If the template is all you have, take its reverse complement first.

The cleanup filter is silent, and that is worth keeping in mind. Anything outside A, U, G and C is dropped without warning: N and the IUPAC ambiguity letters (R, Y, W, S, K, M) simply vanish, the sequence gets shorter and the reading frame slides. Pasting a FASTA header is the nastiest case, because letters such as A, C and T sitting inside a gene name are read as bases — a header naming ACTB comes back as GACU. Spaces, line breaks and the position numbers from an alignment are stripped too, which here is a convenience. Compare output length against input length before you trust the answer. If nothing you typed is a valid base the result box just goes blank instead of reporting an error, and RNA pasted with U already in it returns untouched.