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.

Lesson 6: Transcription — Copying DNA into mRNA