Transcribing DNA to RNA

Transcription copies a gene’s DNA into messenger RNA. On paper it is a simple substitution — but which strand you start from changes what you do, and that is where it catches people out.

Two strands, two rules

DNA is double-stranded and the two strands are named for their relationship to the mRNA:

The coding (sense) strand has the same sequence as the mRNA. To transcribe it, just replace every T with U.
The template (antisense) strand is what RNA polymerase actually reads. To get the mRNA, take itsreverse complement first, then replace T with U.

Worked both ways

Coding strand ATGGCATAG transcribes directly to AUGGCAUAG — identical apart from T→U. The template strand ATGC gives its reverse complement GCAT, then GCAT → GCAU. Same machinery, different starting strand.

Why the coding strand looks like the mRNA

Polymerase reads the template 3′→5′ and builds RNA 5′→3′, base-pairing as it goes. The RNA it makes is therefore complementary to the template — which is the same as the coding strand (also complementary to the template), with U in place of T. That is why gene records show the coding strand: it reads like the message.

Next step: translation

Transcription stops at the RNA sequence. To read the protein it encodes, the mRNA is translated three bases at a time using the codon table — the job of the codon usage tool or the ORF finder. Getting the strand and frame right here is what makes the translation downstream come out correct.