6.2.4—DNA strands in transcription

During transcription, RNA is made from only one DNA strand. The template (transcribed) strand is read to build a complementary mRNA sequence; the non-template (coding) strand is not transcribed and has the same base sequence as the mRNA when T in DNA is replaced by U in RNA.

1. Identify the template: The gene region unwinds and the hydrogen bonds between the DNA strands break. The transcribed/template strand is the one whose exposed bases are used to build the RNA.
2. Build the RNA: RNA polymerase joins complementary RNA nucleotides to the template. The template is read 3′→5′ while the mRNA is built 5′→3′.
3. Use the direction cue: Template DNA 3′–TAC–5′ → mRNA 5′–AUG–3′.
4. Check the coding strand: The non-template/non-transcribed strand is written 5′→3′ and matches the mRNA sequence except that DNA has T where mRNA has U: Coding DNA 5′–ATG–3′ ↔ mRNA 5′–AUG–3′.
5. Place the product: The completed mRNA can leave the nucleus through a nuclear pore and carry the transcript to a ribosome; this is the end of the transcription-focused sequence, before translation is treated in the neighbouring process card.

Only the template strand is complementary to the new RNA, so it determines the mRNA sequence. The coding strand is a useful check because its 5′→3′ sequence matches the mRNA apart from T/U; labelling strand identity and direction prevents complementing the wrong strand twice.

Both DNA strands are present, but only one is transcribed for a given gene. Do not call the coding/non-template strand the template, and do not use T in the mRNA. This card resolves strand identity and direction; the complete DNA→RNA→polypeptide process belongs to card 4578, while intron removal and exon joining belong to card 4580. Staff-only transcription visual brief retained in the Topic Blueprint; no image is generated or bound here.