Your DNA is the master copy of every instruction your body owns, and it is far too precious (and far too big) to leave the nucleus. So the cell does what you would do with a library reference book: it copies out the one page it needs. That copy is mRNA, and making it is called transcription.
📘 What you need to know
Transcription happens in the nucleus and turns one gene into a molecule of mRNA.
The DNA unwinds because the hydrogen bonds between the base pairs break. Only the gene being used is opened up.
Only one of the two DNA strands is copied. That strand is the template strand; the other one is the coding strand.
Free RNA nucleotides pair with the exposed template bases, and the enzyme RNA polymerase joins their sugar–phosphate groups into a backbone.
RNA has no thymine. Wherever DNA would use T, RNA uses uracil (U).
The finished mRNA leaves through a nuclear pore. DNA cannot — it is far too big.
Transcription is the first stage of gene expression, so it is the main place a cell switches a gene on or off.
Why bother making a copy at all?
Proteins are built at ribosomes, and ribosomes sit out in the cytoplasm. The instructions, though, are locked in the nucleus. Something has to carry the message from one place to the other.
DNA itself cannot do it. A whole chromosome is enormous compared with a nuclear pore, and letting the master copy wander around the cell would risk damaging it. So the cell makes a short, disposable, single-stranded copy of just the gene it needs. That copy is messenger RNA — the name is a clue: it is a message being sent somewhere.
Think of the nucleus as a library where the reference books cannot be borrowed. You are allowed to photocopy one page, take the copy to your desk, use it, and throw it away. The book never moves. That photocopy is your mRNA.
Two rooms, two jobs. If a question asks “where”, the answer is almost always nucleus for transcription and cytoplasm for translation.
What actually happens, step by step
🧩 The order of events
Unwind. The hydrogen bonds between the complementary base pairs break, so a short section of the double helix opens up. Only the gene being transcribed is unzipped, not the whole chromosome.
Expose the template. One strand of that opened section is the template strand. Its bases are now free to pair with something new.
Pair up. Free RNA nucleotides floating in the nucleus line up against the template bases and hold on with hydrogen bonds. A pairs with U, T pairs with A, C pairs with G.
Join up.RNA polymerase bonds the sugar–phosphate groups of those nucleotides together, building the backbone of the mRNA molecule.
Zip back up. When the gene has been fully copied, the hydrogen bonds holding the mRNA to the DNA break and the double helix re-forms behind the enzyme.
Leave. The finished mRNA moves out of the nucleus through a nuclear pore and heads for a ribosome.
One enzyme, one job. RNA polymerase does two things at once: it moves along the template reading it, and it stitches the new RNA nucleotides together behind it. The DNA opens in front of it and closes again behind it, like a zip being run along a coat.
Template strand or coding strand?
This is where marks get lost, so slow down here. The two DNA strands are complementary, which means they carry the same information in mirror form. The cell only copies one of them.
The coding strand carries the sequence that matches the eventual mRNA. It is not the one that gets copied.
The template strand is the one RNA polymerase reads. Because the mRNA is complementary to the template, and the template is complementary to the coding strand, the mRNA ends up looking like the coding strand.
So the shortcut is: to write the mRNA, take the coding strand and swap every T for a U. But you should still be able to show the template step if the question asks for it.
Work down the diagram in an exam and you cannot get lost: coding strand, flip it to get the template, flip that to get the mRNA.
The pairing rules you will use every time
Base on the DNA template strand
Base added to the growing mRNA
A (adenine)
U (uracil)
T (thymine)
A (adenine)
C (cytosine)
G (guanine)
G (guanine)
C (cytosine)
Why no thymine in RNA? There is simply no thymine RNA nucleotide floating about for the enzyme to use. Uracil pairs with adenine in exactly the same way, so the message is not damaged — only the spelling changes.
Transcription is the on/off switch
A human genome holds roughly 20,000 protein-coding genes, but no cell needs all of them. A heart muscle cell has the insulin gene sitting in its nucleus and never uses it, because insulin is a pancreas job.
Cells solve this by only transcribing the genes they need. A gene that is being transcribed and translated is expressed (“switched on”); a gene that is not is silenced (“switched off”). Since transcription is the first step of the whole process, blocking it there saves the cell from wasting energy on everything downstream.
If an exam question asks why two cells with identical DNA look and behave completely differently, this is the answer: they are not using the same genes. Same recipe book, different recipes chosen.
Worked examples
WORKED EXAMPLE
From coding strand to mRNA
A short section of the coding strand of a gene reads ATG CCA GTC TTA. Write out the template strand and the mRNA transcript.
Step 1: Build the template strand (DNA pairing: A–T, C–G)ATG CCA GTC TTA → TAC GGT CAG AATStep 2: Transcribe the template into mRNA (A→U, T→A, C→G, G→C)TAC GGT CAG AAT → AUG CCA GUC UUAStep 3: Check it against the coding strand
Same letters as the coding strand, with U in place of every T. That is the sign you have not slipped up.
mRNA = AUG CCA GUC UUADo the template step even if you can see the shortcut. Method marks are free marks.
WORKED EXAMPLE
Working backwards from mRNA
An mRNA transcript begins GCU AAG CCU. Deduce the DNA template strand and the DNA coding strand it came from.
Step 1: The template is complementary to the mRNA (remember U pairs with A)GCU AAG CCU → CGA TTC GGAStep 2: The coding strand is complementary to the templateCGA TTC GGA → GCT AAG CCTStep 3: Sense check
The coding strand matches the mRNA with T for U. It does.
Template = CGA TTC GGA, coding = GCT AAG CCTGoing backwards, U becomes T — there is no uracil in DNA.
WORKED EXAMPLE
Spotting the wrong enzyme
A student writes: “DNA polymerase unzips the DNA and builds the mRNA molecule.” Identify the error and correct it.
Step 1: Find the mistakeDNA polymerase is wrong. That enzyme works in DNA replication, not transcription.
Step 2: Correct it
The enzyme that builds mRNA in transcription is RNA polymerase.
Replace “DNA polymerase” with “RNA polymerase”The clue is in the name: it makes RNA, so it is RNA polymerase.
💡 Exam tip
Name the enzyme every time. “An enzyme joins the nucleotides” will not get the mark. Write RNA polymerase.
Say where. Free marks are given for “in the nucleus” — add it even when the question does not directly ask.
When you write a transcript, keep the bases in groups of three. It costs nothing now and saves you when you have to translate it later.
Two different bond types. Hydrogen bonds hold the bases together; the sugar–phosphate backbone is joined by covalent bonds. Questions do test this difference.
If a question gives you a strand without saying which one it is, look for the word template or coding. Everything hangs on it.
Say the mRNA leaves through a nuclear pore, not “through the membrane”.
⚠ Common mix-up
DNA polymerase vs RNA polymerase. Replication uses DNA polymerase. Transcription uses RNA polymerase. This is the single most common lost mark on this topic.
Copying the wrong strand. The template is read; the coding strand is not. Students often transcribe the coding strand by mistake and get an answer that is the exact complement of the right one.
Leaving a T in the mRNA. RNA has uracil. If you have written a T in an RNA sequence, you have lost the mark.
Saying the DNA “splits in half” completely. Only a short section unwinds — the gene being copied — and it closes again straight afterwards.
Confusing transcription with replication. Replication copies the whole molecule into two DNA molecules. Transcription copies one gene into one strand of RNA.
Saying DNA travels to the ribosome. It does not, and cannot. Only the mRNA makes that trip.
Up next: Translation — the mRNA has arrived in the cytoplasm, so now we watch a ribosome read it three bases at a time and build the actual protein.
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