Coding strand vs template strand in transcription
Why the two DNA strands give different transcription steps, how base pairing drives the mRNA, and which strand to pick in this tool.
The two strands of the double helix
DNA is double stranded, and the two strands run in opposite directions and carry complementary bases. One is called the coding strand, or sense strand, and the other is the template strand, or antisense strand. They hold the same genetic information in mirror form, so knowing one lets you reconstruct the other by pairing A with T and G with C. When you transcribe a gene, only one of these strands is read by the enzyme, which is why picking the right one in this tool matters.
Why the coding strand only needs T to U
The coding strand is named that way because it reads almost exactly like the messenger RNA that the gene produces. The single difference is the alphabet: DNA uses thymine (T) while RNA uses uracil (U). So if you already have the coding strand, transcription is just a find-and-replace that turns every T into a U. That is exactly what the Coding strand option does here, leaving every other base untouched and in the same position.
Why the template strand is complemented first
During real transcription, RNA polymerase reads the template strand and lays down complementary RNA bases. Because the template is the mirror image of the coding strand, the tool first builds its complement (A to U, T to A, G to C, C to G) and then reports that as the mRNA. This is why entering the same letters under Template strand gives a very different answer than under Coding strand. If you are unsure which strand you copied, compare the complementary DNA strand shown below the result to your source.
Choosing the right option in this tool
A practical rule: if your sequence was labelled as the gene, the mRNA sequence, or the sense strand, choose Coding strand. If it was labelled the template, antisense, or the strand being read, choose Template strand. When a textbook shows both strands, the one written 5' to 3' left to right that matches the mRNA is the coding strand. Getting this choice right is the difference between a correct transcript and its complement, so it is worth confirming before you rely on the output.