Transcription — Copying DNA into mRNA

Transcription is the first stage in gene expression: the copying of the sequence of bases in a gene into a complementary mRNA molecule that can be exported from the nucleus to the ribosomes for translation. This lesson covers the OCR A-Level Biology A specification point 2.1.3 (g) — the process of transcription — including the roles of RNA polymerase, the template strand and mRNA synthesis.

A good understanding of transcription requires you to build on concepts from the previous lessons: base pairing (Lesson 2), DNA vs RNA structure (Lessons 2 and 3), and the genetic code (Lesson 5).

1. Why Transcription?

DNA is too precious to let out of the nucleus. It contains the entire genome and must remain intact for the lifetime of the cell. But proteins are made in the cytoplasm (at the ribosomes), so a disposable, single-stranded copy of each gene is made instead: messenger RNA (mRNA). The process of making this RNA copy from a DNA template is called transcription.

Key Definition — Transcription: The process by which a section of DNA (a gene) is used as a template to synthesise a complementary strand of mRNA, catalysed by RNA polymerase.

2. Key Players

Component	Role
DNA gene	The portion of the DNA containing the sequence to be transcribed.
Template strand (antisense)	The strand of DNA that is read by RNA polymerase. The mRNA is complementary to this strand.
Coding strand (sense)	The other strand. Its sequence is (almost) the same as the mRNA, except T is replaced by U.
RNA polymerase	The enzyme that unwinds the DNA, pairs free RNA nucleotides with the template strand and joins them via phosphodiester bonds.
Free RNA nucleotides	Ribonucleoside triphosphates (rATP, rUTP, rCTP, rGTP) that are incorporated into the growing mRNA strand.

Exam Tip: The template strand is read 3' to 5'. The mRNA is built 5' to 3'. The mRNA has the same base sequence as the coding strand (except that U replaces T).

3. The Steps of Transcription

graph TD
  A[DNA gene - double helix] --> B[RNA polymerase binds to promoter]
  B --> C[DNA unwinds; H-bonds break]
  C --> D["RNA polymerase reads template strand<br/>in the 3’ to 5’ direction"]
  D --> E["Free RNA nucleotides pair with<br/>complementary DNA bases<br/>A–U, T–A, C–G, G–C"]
  E --> F["RNA polymerase joins nucleotides<br/>via phosphodiester bonds"]
  F --> G[mRNA strand released]
  G --> H[DNA re-forms double helix]
  H --> I[mRNA leaves nucleus via nuclear pore]

3.1 Initiation

RNA polymerase binds to a specific DNA sequence called the promoter at the start of a gene.
RNA polymerase catalyses the breaking of the hydrogen bonds between complementary bases in the region of the promoter, unwinding the DNA and exposing the template strand. (Unlike replication, no separate helicase is required — RNA polymerase does this itself.)

3.2 Elongation

Free RNA nucleotides (rATP, rUTP, rCTP, rGTP) enter the nucleus and pair with their complementary bases on the template strand according to base-pairing rules.
Complementary base pairing in transcription:
- DNA A pairs with RNA U (not T — remember, RNA has no thymine)
- DNA T pairs with RNA A
- DNA C pairs with RNA G
- DNA G pairs with RNA C
RNA polymerase catalyses the formation of phosphodiester bonds between adjacent ribonucleotides, joining them into a growing mRNA strand.
RNA polymerase moves along the template strand in the 3' to 5' direction, synthesising the mRNA in the 5' to 3' direction.
Behind the polymerase, the two DNA strands re-form their hydrogen bonds and the DNA double helix is restored.

3.3 Termination

When RNA polymerase reaches a specific stop signal (a termination sequence) on the DNA, transcription ends.
The completed mRNA molecule is released.
The DNA rewinds and the two strands re-form a double helix.
RNA polymerase detaches.

In eukaryotes, the newly made mRNA is then processed (adding a cap and tail and removing non-coding regions called introns), but this is not required by the OCR A-Level Biology A specification and you only need to know the outline above.

4. Worked Example

Given the following DNA:

Strand	Direction	Sequence
Coding (sense)	5' → 3'	ATG CAT TTC GGA TAA
Template (antisense)	3' → 5'	TAC GTA AAG CCT ATT

The template strand is read 3' to 5'. The mRNA is built 5' to 3' and is complementary to the template:

Product	Direction	Sequence
mRNA	5' → 3'	AUG CAU UUC GGA UAA

Notice that:

The mRNA sequence is the same as the coding strand, except T is replaced by U.
This is why the coding strand is sometimes called the "sense" strand: its sequence matches the mRNA.

5. Comparing Transcription with Replication

Feature	Replication (Lesson 4)	Transcription
Purpose	To copy the entire genome for cell division	To make an RNA copy of a single gene
Product	Two complete double-stranded DNA molecules	One single-stranded mRNA molecule
Enzyme	DNA polymerase (+ helicase + others)	RNA polymerase
Bases used	A, T, C, G	A, U, C, G
Amount of DNA unwound	Entire genome	Just one gene at a time
Template	Both strands	Only one strand (the template)
Error rate	Very low (proofreading)	Higher (no proofreading) — acceptable because mRNA is disposable

Exam Tip: Students often confuse the enzymes. DNA polymerase builds new DNA strands; RNA polymerase builds new RNA strands.

6. Why Transcription Is Useful

Controls gene expression — genes can be switched on or off by regulating RNA polymerase, allowing cells to make different proteins in response to different conditions.
Amplification — one gene can be transcribed many times, producing many mRNA molecules, and each mRNA can be translated multiple times. A single gene can therefore produce thousands of protein molecules without any change to the DNA.
Protects the genome — only a disposable RNA copy leaves the nucleus; the DNA is never removed.

7. Common Exam Mistakes

Confusing template and coding strands. The template strand is the one read by RNA polymerase; the coding strand has (almost) the same sequence as the mRNA.
Forgetting that DNA A pairs with RNA U (not T) during transcription.
Writing that "DNA polymerase" transcribes. It is RNA polymerase.
Forgetting to state that phosphodiester bonds are formed between the ribonucleotides.
Saying the whole DNA is unwound. Only a small region around the gene is unwound at any one time.
Failing to mention complementary base pairing and the role of RNA polymerase in catalysing the condensation reactions.

8. Exam-Style Questions

Describe the process of transcription in a eukaryotic cell. (6)
A section of a template strand of DNA has the base sequence 3'-TACGTAAAG-5'. Write the base sequence of the mRNA produced. (1)
Explain why RNA polymerase reads the template strand in the 3' to 5' direction. (2)
State two differences between DNA replication and transcription. (2)

Model answer for (2): "5'-AUGCAUUUC-3'."

Summary

Transcription makes a single-stranded mRNA copy of a gene.
RNA polymerase binds to the promoter, unwinds the DNA and breaks the hydrogen bonds.
The template strand is read in the 3' to 5' direction, and mRNA is built in the 5' to 3' direction.
Complementary base pairing (DNA A–RNA U, DNA T–RNA A, DNA C–RNA G, DNA G–RNA C) ensures the mRNA carries a correct copy of the genetic information.
Free RNA nucleotides are joined by phosphodiester bonds formed via condensation.
Transcription ends at a stop signal; the mRNA is released; the DNA rewinds.
The mRNA leaves the nucleus and travels to the ribosomes for translation.

A-Level Deep Dive

Spec mapping

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.3 — Nucleotides and nucleic acids, covering the synthesis of mRNA from a DNA template by transcription, including the role of RNA polymerase and the directional reading of the template strand (refer to the official OCR H420 specification document for exact wording).

Transcription is the first step of gene expression and is a recurrent assessment object on Paper 1 and synoptically on Paper 3. The combination of mechanism + comparison with replication is a common 6-mark structure. The clean separation of transcription (nucleus, RNA polymerase) from translation (cytoplasm, ribosome) underlies the eukaryotic cell-architecture content of Module 2.1.1, so this lesson is the bridge between cell structure and molecular biology.

Scientists and paradigms

The messenger-RNA hypothesis — that an unstable intermediate ferries genetic information from DNA to ribosome — was proposed in 1961 by François Jacob, Sydney Brenner and Matthew Meselson. Their experiment used phage-infected E. coli labelled with ¹⁵N to show that newly made protein was assembled by old ribosomes reading a new mRNA template — the mRNA, not the ribosome, was the species that turned over rapidly.

Roger Kornberg (son of Arthur Kornberg) won the 2006 Nobel Prize in Chemistry for crystal structures of RNA polymerase II — atomic-resolution images of the enzyme caught in the act of transcribing DNA. The school of thought to take into the exam: transcription is enzymatic copying with template-directed polymerisation, mechanistically parallel to DNA replication but with a single-strand product, ribose sugar and uracil base.

Paraphrase, do not invent quotation: the schools of thought are that "mRNA is an unstable working copy" (Jacob, Brenner, Meselson) and that "RNA polymerase is a molecular machine reading a duplex template" (Kornberg).

Synoptic links

This lesson connects forward to:

ocr-alevel-biology-nucleic-acids-enzymes — Translation (Lesson 7): the mRNA produced here is the template for protein synthesis. The 5'→3' directionality you establish here determines the direction of ribosomal scanning.
ocr-alevel-biology-genetics-inheritance — Gene expression and regulation: transcription factors, promoters, enhancers and operons (e.g. lac operon in E. coli) all regulate the rate of transcription.
ocr-alevel-biology-cell-structure: in eukaryotes, transcription occurs in the nucleus; mRNA processing (capping, splicing, polyadenylation) happens there before export through nuclear pores.
ocr-alevel-biology-nucleic-acids-enzymes — DNA replication (Lesson 4): transcription and replication share the principle of template-directed synthesis by complementary base pairing, but differ in product (RNA vs DNA), enzyme (RNA polymerase vs DNA polymerase), extent (one gene vs whole genome), and proofreading (none vs 3'→5' exonuclease).
ocr-alevel-biology-photosynthesis-respiration — ATP: ribonucleoside triphosphates (ATP, UTP, GTP, CTP) are the energy-rich precursors for RNA synthesis, paralleling dNTPs in DNA synthesis.

Specimen question modelled on the OCR H420 paper format

Question (6 marks): Describe the process of transcription, and explain how transcription differs from DNA replication.

Mark scheme decomposition (AO breakdown):

Mark	AO	Awarded for
1	AO1	RNA polymerase binds to a promoter region of DNA
2	AO1	Helix is unwound; hydrogen bonds between the two strands are broken
3	AO1	Template strand read 3'→5'; complementary RNA nucleotides align by base pairing (A–U, T–A, G–C, C–G)
4	AO1	RNA polymerase joins RNA nucleotides by phosphodiester bonds; mRNA built 5'→3'
5	AO2	Difference 1: transcription is single-stranded product / uses ribose / U instead of T
6	AO2	Difference 2: transcription copies one gene only; replication copies entire genome; transcription uses RNA polymerase, replication uses DNA polymerase

Split: AO1 = 4, AO2 = 2.

Grade C model answer (~165 words)

In transcription, RNA polymerase binds to the promoter on the DNA. The DNA double helix is unwound and the hydrogen bonds between the bases are broken. Only one of the two strands — the template strand — is used to make the mRNA. Free RNA nucleotides line up against the template strand by complementary base pairing. A pairs with U (not T because there is no thymine in RNA), and C pairs with G. RNA polymerase joins the RNA nucleotides together by phosphodiester bonds, building the mRNA in a 5' to 3' direction. The mRNA is single-stranded and is released, then it leaves the nucleus.

Transcription is different from DNA replication because (i) only one strand is used as template, and the product is single-stranded mRNA, not double-stranded DNA; (ii) RNA polymerase is used instead of DNA polymerase, and ribose replaces deoxyribose, with uracil instead of thymine.

Lesson 6: Transcription — Copying DNA into mRNA