Finding where to start
A genome is a very long string of letters, and most of it is not the gene you want right now. So how does the cell know where a gene begins? Just upstream of each gene sits a short signpost sequence called a promoter. It is not copied into the message itself; it is a landing pad that says “a gene starts here — read this way.”
The enzyme that does the reading is RNA polymerase. It recognizes the promoter, clamps onto the DNA there, and opens a small bubble in the double helix so it can read the bases inside. The promoter does double duty: it marks the start point and it sets the direction of reading.
One strand is the template, the other is the copy
Only one of the two DNA strands is read for any given gene — the template strand. RNA polymerase reads it and lays down RNA bases by base pairing, the same matching trick as in replication, with one twist: RNA has no thymine. Where the template says A, the RNA gets U (uracil) instead of T. Everything else pairs as usual: C with G, G with C.
TRANSCRIPTION — read the template, write the RNA
coding strand 5'-A T G G A C T T C-3' (looks like the mRNA)
template strand 3'-T A C C T G A A G-5' ◀ this strand is READ
│
▼ RNA polymerase pairs each base (A→U)
messenger RNA 5'-A U G G A C U U C-3'
Check: template T→A, A→U, C→G, C→G, T→A, G→C ...
Result mRNA matches the coding strand, with U everywhere it had T.The copy heads out
When the polymerase reaches a stop signal at the gene's end, it releases the finished strand. That strand is the messenger RNA — the working copy from Guide 1, now actually made. The whole reading process is transcription, and the amount of mRNA a cell chooses to make from each gene is a major lever of gene expression.
Now the message is ready to be decoded. But before we can read it as a protein, we need the rulebook that turns RNA letters into amino acids — the genetic code. That is the next guide.