IB Biology SLTopic 4 — Making ProteinsPaper 1 & 2Core idea~8 min read
Transcription: Making an mRNA Copy
Your DNA never leaves the nucleus. So when a cell needs a protein, it does not send the DNA anywhere — it makes a short, throw-away copy of just the one gene it needs. That copy is mRNA, and making it is called transcription.
📘 What you need to know
Making a protein takes two stages: transcription (DNA → mRNA, in the nucleus) and then translation (mRNA → polypeptide, at a ribosome).
Only one of the two DNA strands is copied. That strand is the template strand.
Free RNA nucleotides pair with the exposed DNA bases. C still pairs with G, but A on the DNA pairs with U on the RNA, because RNA has no thymine.
RNA polymerase joins the RNA nucleotides together to build the sugar–phosphate backbone of the mRNA.
mRNA is single-stranded and small enough to fit through a nuclear pore. DNA is not.
A cell only transcribes the genes it actually needs — this is gene expression.
Why bother making a copy?
Think of your DNA as the reference section of a library. Those books are far too valuable to lend out, so you photocopy the one page you need and take the copy away with you. The original stays safely on the shelf.
The cell does exactly the same thing, for two reasons:
DNA is too big to leave the nucleus. It cannot fit through the pores in the nuclear envelope. mRNA is much shorter and single-stranded, so it slips straight through.
The original stays protected. Ribosomes are out in the cytoplasm, which is a busy place. Sending a disposable copy means the master set of instructions is never at risk.
If a question asks why the mRNA leaves but the DNA does not, examiners want the size point: DNA is too large to pass through a nuclear pore. Say it in those words and the mark is yours.
Transcription in one line
gene on the DNA template strand → mRNA (built by RNA polymerase)
What actually happens
🧩 The process, step by step
The helix unwinds. Part of the DNA molecule opens up. The hydrogen bonds between the complementary base pairs break, so the two strands separate and the bases of the gene are exposed.
One strand is chosen. Only the template strand is read. The other strand just sits there.
RNA nucleotides move in. Free RNA nucleotides floating in the nucleus pair up with the exposed bases by hydrogen bonding, following the base pairing rules.
The backbone is joined up. RNA polymerase bonds the sugar–phosphate groups of those RNA nucleotides together, so they become one continuous mRNA molecule.
The helix zips back up. Once the whole gene has been copied, the hydrogen bonds between the mRNA and the DNA break, the mRNA peels away, and the double-stranded DNA re-forms behind the enzyme.
The mRNA leaves. It travels out through a nuclear pore into the cytoplasm and heads for a ribosome.
RNA polymerase works its way along the gene like a zip. Everything behind it has already closed up again, which is why the rest of the chromosome is never left hanging open.
Which strand gets copied?
A DNA molecule has two strands, and they are not interchangeable here. Only one is used as the pattern.
Strand
What it does
Coding strand
Carries the genetic code in the same order as the mRNA will. It is not transcribed — it is just the partner strand sitting opposite.
Template strand
The one that is actually read. RNA nucleotides pair with its exposed bases, so the mRNA ends up complementary to it.
mRNA transcript
Complementary to the template strand, which makes it a match for the coding strand — except every T has become a U.
Two strands, two jobs. Reading the wrong one is the single most common way students lose marks on this topic.
The base pairing rules change slightly
RNA does not contain thymine at all. Wherever a T would have gone, a U goes instead. Everything else pairs as normal.
Base on the DNA template strand
RNA base added to the mRNA
A (adenine)
U (uracil)
T (thymine)
A (adenine)
C (cytosine)
G (guanine)
G (guanine)
C (cytosine)
Fast check for exams: the finished mRNA should read exactly like the coding strand, with U in place of every T. If it does not, you have copied the wrong strand or made a pairing slip.
Not every gene is switched on
There are roughly 20 000 protein-coding genes in a human cell, and every one of your cells carries the whole set. But a heart muscle cell has no use for insulin, and a pancreas cell has no use for muscle proteins. So cells switch genes on and off to suit the job they do. That is what gene expression means.
A gene that is switched on gets transcribed and then translated, so its protein gets made.
A gene that is switched off (silenced) is not transcribed at all, so no protein appears.
Transcription is the first stage of gene expression, which makes it the obvious place for the cell to control things. Block transcription and nothing else can happen.
Worked examples
WORKED EXAMPLE
Write the mRNA made from a template strand
A section of the template strand of a gene reads T A C G G A A G T. Write down the sequence of the mRNA transcribed from it.
Step 1: Note which strand you have been given
This is the template strand, so the mRNA is complementary to it.
Step 2: Pair each base, using U instead of TT → A A → U C → G G → CStep 3: Read it back in orderA U G C C U U C ACareful with A on the DNA — it pairs with U, never with T.
WORKED EXAMPLE
Start from the coding strand instead
The coding strand of a gene reads G A A T C G. Write the mRNA transcribed from this gene, and explain your method.
Step 1: Work out the template strand firstCoding: G A A T C G → Template: C T T A G CStep 2: Transcribe the template into mRNAC → G, T → A, T → A, A → U, G → C, C → GG A A U C GShortcut: it is the coding strand with T swapped for U. Doing the long way once shows you why the shortcut works.
WORKED EXAMPLE
Explain the difference for 3 marks
State three ways the mRNA molecule differs from the DNA molecule it was copied from.
Point 1
mRNA is single-stranded; DNA is double-stranded.
Point 2
mRNA contains uracil instead of thymine.
Point 3
mRNA is much shorter — it is a copy of one gene, not the whole molecule.
A fourth one if you need it: the sugar in RNA is ribose, in DNA it is deoxyribose.
💡 Exam tip
Read the strand label twice. “Coding strand” and “template strand” look similar on a rushed reading, and the answers are completely different.
Write the bases in spaced-out groups of three when you copy them out. It stops slips and sets you up for reading codons later.
Say RNA polymerase, not “polymerase”. The name of the enzyme is worth a mark on its own.
If asked where transcription happens, answer in the nucleus. It is a one-word mark that students throw away.
Mention hydrogen bonds when describing base pairing, and sugar–phosphate backbone when describing what the enzyme joins. These are the two phrases mark schemes look for.
Check your final mRNA against the coding strand. Same letters, U for T. A five-second check that catches most errors.
⚠ Common mix-up
DNA polymerase and RNA polymerase. DNA polymerase copies DNA during replication. RNA polymerase builds mRNA during transcription. Different jobs, different names.
Transcribing the coding strand. The coding strand is not read. If you transcribe it, every base in your answer will be wrong.
Writing T in an mRNA answer. RNA has no thymine. One stray T can cost the mark.
Saying the DNA “turns into” mRNA. It does not. The DNA is copied and left completely unchanged.
Thinking the whole chromosome is transcribed. Only the gene being expressed is opened up and copied.
Mixing up transcription and translation. Transcription = writing the same language (nucleic acid to nucleic acid). Translation = changing language (nucleic acid to amino acids).
Up next: Translation — what happens when that mRNA reaches a ribosome and the message finally gets turned into a chain of amino acids.
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