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D1.2.1—Transcription

Transcription produces an RNA copy of gene information when RNA polymerase uses one DNA strand as a complementary template for mRNA synthesis.

Syllabus
First assessment 2025
Objective
D1.2.1
Level
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–4

Common command terms

  • State
  • Outline
  • Explain
  • Describe
  • Distinguish

Scoring notes

Common mistake
Confusing transcription with translation by naming ribosomes or amino acids instead of RNA polymerase and mRNA.

Recent exam appearances

May 2025Paper2 ["SL"] · TZ37(b)[ 4 ]D1.2.1—Transcription
November 2024Paper1 ["SL"] · TZ17[ 1 ]D1.2.1—Transcription
May 2019Paper2 ["SL"] · TZ17(a)[ 4 ]D1.2.1—Transcription
May 2015Paper1 ["SL"] · TZ111[ 1 ]D1.2.1—Transcription
November 2014Paper1 ["SL"] · TZ09[ 1 ]D1.2.1—Transcription

Coverage 2012–2025 · Updated 16 Jul 2026

mRNA Carries a Working Copy of Selected DNA Information

Chromosomal DNA is the long-term information store. Protein synthesis instead uses a short-lived messenger RNA (mRNA) copy of a selected gene.

selected gene in DNA → complementary mRNA copy → mRNA reaches a ribosome → nucleotide sequence directs amino-acid order

A working copy solves two problems:

  • The original DNA sequence remains protected and reusable.
  • Many mRNA copies can be made, so one gene can support repeated protein production.

mRNA carries information; it is not converted into protein matter. The ribosome reads its sequence while amino acids are joined into a separate molecule.

RNA Polymerase Builds mRNA from One DNA Template Strand

1

RNA polymerase binds near the gene and opens a short region of the DNA double helix, exposing the template strand.

RNA polymerase opens a short DNA region, reads the template strand and synthesizes an mRNA strand that leaves the transcription bubble.
2

Free RNA nucleotides align by complementary base pairing: DNA A pairs with RNA U, DNA T with RNA A, and C with G.

3

RNA polymerase joins the RNA nucleotides into a sugar–phosphate backbone. The mRNA separates, and the DNA strands pair again.

Template DNA Determines the mRNA Sequence

Strand Relationship to the new mRNA
DNA template antiparallel and complementary
DNA coding strand same base order as mRNA, except DNA has T where RNA has U

DNA template: 3′–TAC CTT GCG–5′

mRNA: 5′–AUG GAA CGC–3′

Codon tables are read from mRNA 5′→3′. Do not look up a DNA template triplet directly.

DNA template bases pair with RNA nucleotides according to A–U and C–G complementarity.

Reconstruct the Route from a Selected Gene to mRNA

gene selected → DNA opens locally → RNA polymerase reads one template strand → complementary RNA nucleotides are joined → mRNA leaves → DNA re-forms unchanged

Question Correct check
Which DNA strand determines mRNA? the template strand
Which base replaces thymine in RNA? uracil
Which direction is mRNA written? 5′→3′
Why can the gene be reused? DNA sequence and covalent backbones remain intact

The mRNA now carries codons in an order that a ribosome can translate into an amino-acid sequence.

Transcription exam focus

[ 8 ]

Explain the process of transcription in prokaryotes.

Concept essentials

  • Transcription copies one DNA strand into an RNA molecule.
  • RNA polymerase binds DNA, separates the strands and joins RNA nucleotides.
  • The antisense strand provides the template for complementary RNA base pairing.
  • Eukaryotic transcription occurs in the nucleus and can be followed by RNA processing.