Transcription, From DNA to mRNA
You'll be able to convert a DNA template sequence into an mRNA sequence correctly.
In 1961, Marshall Nirenberg and Heinrich Matthaei at the National Institutes of Health used a cell-free system to test synthetic poly-U RNA and found it produced only the amino acid phenylalanine, proving that the codon UUU codes for Phe. This cracked the first entry in the genetic code. By 1966, all 64 codons had been decoded, and the code was found to be near-universal: the same 64 codon assignments operate in bacteria, plants and humans, with only a handful of known exceptions such as vertebrate mitochondria. That near-universal pattern is only possible because transcription copies DNA information into mRNA using the same base-pairing rules in every organism.
Video companions
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Trace a DNA Sequence or a Meiosis Cell: HSC-Style Practice | Year 12 Biology
Practice companion Duration: 8 minutes 48 seconds
Orient yourself
Predict why a cell copies DNA into mRNA rather than moving the DNA, then scan the goals and key terms.
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
A student says, "If DNA already contains the code, the cell should just send the DNA out to the ribosome whenever it needs a protein. There is no real reason to make mRNA first."
Before reading on, explain why cells use mRNA rather than moving the DNA itself. What advantage does a temporary RNA copy give the cell?
Know
- A gene is a DNA sequence that codes for a product.
- Transcription forms mRNA from a DNA template strand.
Understand
- Why mRNA is needed as a temporary copy rather than moving DNA itself.
- How codons preserve the code in transferable form.
Can Do
- Convert a DNA template sequence into an mRNA sequence correctly.
- Explain transcription using the CFTR gene as a disease-relevant example.
What a gene actually is
Establish that a gene is a coded DNA sequence and that its base order carries the information.
Core Content
Gene concept · the order of bases carries information
When Marshall Nirenberg and Heinrich Matthaei set up their 1961 experiment, they added synthetic poly-U RNA to a mixture of ribosomes, transfer RNAs, amino acids, and energy molecules, everything needed for protein synthesis except the messenger. The poly-U RNA they added was entirely artificial, not copied from any genome. Yet the ribosomes read it and assembled chains of phenylalanine. This demonstrated that the codon sequence in mRNA, not the DNA itself, is what the ribosome reads. Transcription, the production of that mRNA copy from a specific gene, is therefore the step that converts stored DNA information into a readable, portable message.
A gene is a section of DNA containing the base sequence information needed to produce a functional product. In the HSC Biology context for this module, the important link is that genes contain the code that will later direct polypeptide synthesis.
The base sequence matters because the order of bases carries information. That information must be copied into a usable form for the next step of protein production.
A gene is a section of DNA whose base sequence codes for a functional product, usually a polypeptide. The order of bases carries the hereditary information. A gene "codes for" a product, it is not itself a trait.
Pause, copy the highlighted gene definition into your book before moving on.
A section of DNA that codes for a functional product (usually a polypeptide) is called a _____.
RNA polymerase, the promoter and the strands
See how RNA polymerase binds the promoter and reads the template strand to build a complementary mRNA.
Transcription steps · complementary pairing with uracil
We just saw that genes carry information as a base sequence in DNA. That raises a question: how does that information get copied into a usable form outside the nucleus? This card answers it → transcription uses one DNA template strand to build a complementary mRNA sequence.
Transcription is carried out by the enzyme RNA polymerase. It begins by binding to a control sequence just before the gene called the promoter, which tells the enzyme where a gene starts and which strand to read. RNA polymerase then unwinds the DNA and one strand acts as the template strand (also called the antisense strand). RNA nucleotides pair with the exposed DNA bases using complementary base pairing rules, except that RNA uses uracil instead of thymine.
The two DNA strands run in opposite directions, so direction matters. RNA polymerase reads the template strand in the 3′ to 5′ direction and builds the new mRNA in the 5′ to 3′ direction. Because of this pairing, the mRNA sequence matches the coding strand (also called the sense strand), the DNA strand that is not read, except that mRNA carries uracil wherever the coding strand has thymine.
If the DNA template strand has adenine, the mRNA formed will contain uracil. If the template has thymine, the mRNA formed will contain adenine. Cytosine still pairs with guanine, and guanine still pairs with cytosine.
Once the RNA sequence is formed, the mRNA separates and carries the coded information away from the DNA.
RNA polymerase binds the promoter, unwinds the DNA and reads the template (antisense) strand 3′→5′, building mRNA 5′→3′. RNA nucleotides pair complementarily, A pairs with U, T pairs with A, C with G, G with C; RNA uses uracil instead of thymine. The mRNA matches the coding (sense) strand, the untranscribed strand. Only ONE DNA strand is the template for any given gene.
Add the highlighted transcription rule to your notes, including the base-pairing substitution.
In RNA, which base replaces thymine and pairs with adenine?
Following the copy out of the nucleus
See where transcription sits in the central dogma and why mRNA is a temporary, portable copy.
Transcription (this lesson) copies a gene into mRNA; translation (next lesson) builds the protein.
Why mRNA matters · protecting the DNA
We just saw that transcription copies a DNA template strand into mRNA using complementary base pairing. That raises a question: why bother making a copy, why can't the ribosome just read the DNA directly? This card answers it → mRNA acts as a temporary, portable messenger so the original DNA never needs to leave the nucleus.
In eukaryotic cells, DNA remains in the nucleus. mRNA is important because it acts as a temporary copy of the gene that can be used outside the nucleus in the next stage of polypeptide synthesis.
This protects the original DNA from having to move around the cell each time a protein is needed. It also allows the cell to make multiple RNA copies from the same gene if many copies of a protein are required.
mRNA therefore does not replace DNA. It carries the relevant information from DNA in a form that can be used by the cell.
In eukaryotes, DNA stays in the nucleus. mRNA is a temporary, portable copy of the gene that carries the coded message out of the nucleus. This protects the DNA and allows many protein copies from one gene. mRNA does NOT replace DNA.
Pause, write the highlighted mRNA role into your book.
After transcription, the mRNA permanently replaces the cell's DNA.
During transcription, RNA polymerase synthesises mRNA using the template (antisense) strand of DNA.
Transcription produces a DNA copy of an RNA template for protein synthesis.
Reading the message in threes
See how mRNA carries codons, and where this lesson stops before translation.
Transferable information · DNA vs mRNA bases
We just saw that mRNA is a temporary portable copy that carries the gene's information out of the nucleus. That raises a question: in what units is that information organised on mRNA? This card answers it → mRNA is read in three-base units called codons, with U replacing T compared to DNA.
The sequence on mRNA is read in groups of three bases called codons. At this lesson stage, the key idea is that codons hold transferable information copied from DNA. In the next lesson, you will see how those codons are used in translation.
DNA During Transcription
- Stays as the original hereditary material
- One strand acts as the template
- Uses bases A, T, C and G
mRNA During Transcription
- Temporary copy of the coded sequence
- Forms by complementary pairing to the template
- Uses bases A, U, C and G
mRNA is read in three-base units called codons. Codons hold the transferable information copied from DNA. DNA uses bases A, T, C, G; mRNA uses A, U, C, G. How those codons are decoded into a polypeptide (translation) is the next lesson.
Add the highlighted codon definition and the DNA/mRNA base list to your notes.
A three-base unit on mRNA that carries coded information is called a:
CFTR, base by base
Work the four-step model and the sequence tasks, then apply it to the CFTR gene.
Model · the CFTR gene as a real example
We just saw that mRNA carries codons, three-base units copied from one DNA template strand. That raises a question: what does the full transcription process look like as a sequence of steps? This card answers it → a four-step model using the CFTR gene as a worked example.
The CFTR gene is one real example where the DNA sequence matters biologically. Before any CFTR protein can be produced, the gene must first be transcribed into mRNA.
Transcription: Step 1, the DNA region unwinds, exposing the template strand. Step 2, RNA nucleotides pair with exposed bases (A-U, C-G). Step 3, the mRNA strand separates as a temporary copy. Step 4, the mRNA carries codons out of the nucleus for translation.
Pause, write the four-step transcription sequence into your book in your own shorthand.
Transcription uses one DNA template strand to build a complementary mRNA sequence.
Activities
Sequence and Annotate
For the DNA template strand T A C C G A A T T, write the complementary mRNA sequence in codons. Then label which sequence is DNA and which is mRNA.
CFTR Transcription Reasoning
Explain why a change in the DNA sequence of the CFTR gene could change the mRNA produced during transcription, even before translation happens.
Core idea
- Transcription copies the information in a gene from DNA into mRNA.
Mechanism / process
- One DNA template strand guides complementary pairing of RNA nucleotides to form mRNA, which carries codons.
Common mistake
- Do not say the cell moves DNA to the ribosome or that both DNA strands are copied into one mRNA.
Exam sentence starter
- "mRNA is important in transcription because it acts as..."
Assess the importance of mRNA in transcription and in the overall process of polypeptide synthesis.
Assess asks for a judgement of importance against criteria — how essential mRNA is and what depends on it, not merely what it does.
Plan: criteria, the case for its importance, the limits of that importance, then a verdict.
The importance of mRNA will be assessed against three criteria: whether the process could occur without it, what it contributes beyond carrying information, and how much of the control of gene expression depends on it.
Against the first criterion mRNA is indispensable. DNA cannot leave the nucleus and ribosomes are in the cytoplasm, so without an intermediate the information could not reach the site of synthesis at all. mRNA is transcribed as a complementary copy of the template strand, so it carries the base sequence faithfully in a form that can be exported. Because it is a disposable copy rather than the original, the DNA is never exposed to the cytoplasm or to the mechanical stress of translation, which protects the permanent record.
Against the second and third criteria its importance is greater still. One gene can be transcribed into many mRNA molecules, each translated many times, so mRNA provides amplification — a single copy of a gene can produce enormous quantities of protein. Its quantity and lifespan are also regulated, so controlling how much mRNA is made and how long it survives is one of the principal ways a cell controls how much protein it produces.
Its importance is nonetheless partial rather than total. mRNA carries information but performs no catalysis and cannot decode itself: it requires tRNA to match codons to amino acids and ribosomal RNA to catalyse peptide bond formation. In eukaryotes the primary transcript is also non-functional until introns are spliced out, so mRNA depends on processing before it is usable.
On balance mRNA is essential and irreplaceable in its role but insufficient on its own: it is the only molecule that carries the sequence from nucleus to ribosome, and it is the principal point at which expression is regulated, yet it produces nothing without tRNA and rRNA. Its importance lies in being the controllable step rather than the active one.
The importance of mRNA will be assessed against whether synthesis could occur without it, what it contributes beyond carrying information, and how much control of gene expression depends on it. It is indispensable on the first: DNA cannot leave the nucleus and ribosomes are cytoplasmic, so without an intermediate the information could not reach the site of synthesis; mRNA is transcribed as a complementary copy of the template strand and carries the sequence faithfully in an exportable form; and because it is a disposable copy the DNA is never exposed to the cytoplasm or the stress of translation, protecting the permanent record. On the second and third its importance is greater still, since one gene can be transcribed into many mRNA molecules each translated many times, providing amplification, while its quantity and lifespan are regulated, making mRNA one of the principal points at which a cell controls how much protein it produces. Its importance is nonetheless partial: mRNA performs no catalysis and cannot decode itself, requiring tRNA to match codons to amino acids and ribosomal RNA to catalyse peptide bonds, and in eukaryotes the primary transcript is non-functional until introns are spliced out. On balance mRNA is essential and irreplaceable in its role but insufficient alone — the only molecule carrying the sequence from nucleus to ribosome and the principal point of regulation, yet producing nothing without tRNA and rRNA. Its importance lies in being the controllable step rather than the active one.
Explain how the structure of the transcribed mRNA is determined by the DNA template strand.
Explain asks for the mechanism by which one sequence specifies another. The marks are for the determination, not for a narrative of transcription.
Plan: name the principle, follow the mechanism through the steps, then answer directly.
The mRNA sequence is determined rather than chosen, because RNA polymerase adds each nucleotide by complementary base pairing with the template strand. Only one strand of the DNA is read, so the transcript is specified unambiguously by that strand's sequence.
RNA polymerase binds at a promoter region upstream of the gene, and the promoter's orientation determines which strand is the template and in which direction transcription proceeds. This means the same DNA sequence would specify a different transcript if read from the other strand, so strand selection is part of what determines the product.
As the polymerase moves along the template it adds ribonucleotides complementary to each template base — cytosine opposite guanine, guanine opposite cytosine, adenine opposite thymine, and uracil rather than thymine opposite adenine. Because each pairing is determined by the template base, the growing transcript is a complementary copy of the template and therefore identical in sequence to the coding strand, with uracil replacing thymine.
In eukaryotes the primary transcript is then processed: introns are excised and exons joined, a cap and a poly-A tail are added. This means the final mRNA is determined by the template sequence together with which exons are retained, so a single gene can specify more than one mature transcript through alternative splicing.
The mRNA sequence is determined rather than chosen, because RNA polymerase adds each nucleotide by complementary base pairing with the template strand, and only one strand is read so the transcript is specified unambiguously. RNA polymerase binds at a promoter upstream of the gene, and the promoter's orientation determines which strand is the template and in which direction transcription proceeds, so the same DNA would specify a different transcript if read from the other strand. As the polymerase moves along the template it adds ribonucleotides complementary to each template base — cytosine opposite guanine, guanine opposite cytosine, adenine opposite thymine, and uracil rather than thymine opposite adenine — so the transcript is complementary to the template and therefore identical in sequence to the coding strand, with uracil replacing thymine. In eukaryotes the primary transcript is then processed, introns excised and exons joined with a cap and poly-A tail added, so the final mRNA is determined by the template sequence together with which exons are retained — which is how a single gene can specify more than one mature transcript through alternative splicing.
Practise independently
Attempt every response in your own words first. The model answers are for checking, not for copying.
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Define transcription and outline the role of the DNA template strand.
AnalyseBand 4(4 marks) 2. Explain why mRNA is required as a temporary copy of a gene in eukaryotic cells.
EvaluateBand 5–6(5 marks) 3. Evaluate the statement: "A change in the CFTR DNA sequence can affect the cell even before translation, because transcription depends on the DNA base order."
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1, Sequence and Annotate
DNA template strand: T A C C G A A T T
mRNA formed: A U G G C U U A A
Activity 2, CFTR Transcription Reasoning
If the DNA sequence of the CFTR gene changes, the complementary mRNA sequence produced during transcription can also change. That means the codons carried by the mRNA may differ before translation even begins.
Short Answer Model Responses
Q1 (3 marks): Transcription is the process of producing an mRNA copy from a DNA template strand [1]. The DNA unwinds and one strand acts as the template [1]. Complementary RNA nucleotides pair with that template to form the mRNA sequence [1].
Q2 (4 marks): mRNA is required because DNA remains in the nucleus in eukaryotic cells [1]. The cell therefore needs a temporary copy of the gene that can carry the coded information away from the DNA [1]. mRNA performs this role by holding the copied sequence in transferable form [1]. This protects the original DNA and allows the code to be used in the next stage of polypeptide synthesis [1].
Q3 (5 marks): The statement is valid because transcription depends directly on the DNA base order [1]. During transcription, RNA nucleotides pair complementarily with the DNA template strand [1]. If the CFTR DNA sequence changes, the mRNA sequence produced can also change [1]. That means the codons carried by the mRNA may differ before translation begins [1]. Therefore a DNA sequence change can affect cell function at the transcription stage by altering the copied message [1].
Retrieve and reflect
Use the review session first, then compare how your thinking has changed since the opening prediction.
Gene
A DNA sequence containing coded information for a product.
Transcription
Formation of mRNA from one DNA template strand.
mRNA
A temporary, portable copy of the code.
Exam trap
mRNA carries codons, but translation is the next lesson.
Rapid-fire questions on genes, the DNA template strand, mRNA, uracil and codons. Beat the boss to bank a tier, gold (perfect + fast), silver (80%+), or bronze (cleared).
Nirenberg and Matthaei's 1961 NIH experiment, using synthetic poly-U RNA to identify UUU as the codon for phenylalanine, launched the complete decoding of all 64 codons by 1966. The finding that the genetic code is near-universal (bacteria, plants and humans use the same 64 codon assignments, with only a few known exceptions such as vertebrate mitochondria) is the molecular consequence of the transcription mechanism: RNA polymerase copies a DNA template strand using the same complementary base-pairing rules (A pairs with U, T pairs with A, C pairs with G) in every living organism. The cell cannot send DNA directly to ribosomes, DNA stays in the nucleus as a protected master copy, and transcription produces the temporary mRNA messenger that carries the codon sequence to the ribosome for translation.