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D1.2.4—Transcription for gene expression

Transcription controls gene expression by producing mRNA only from genes activated in particular cells, tissues, stages or environmental conditions within organisms.

Syllabus
First assessment 2025
Objective
D1.2.4
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2019
Most common paperPaper2
Typical marks1–2

Common command terms

  • Determine
  • Outline
  • Compare
  • Deduce
  • Evaluate

Scoring notes

Common mistake
Treating a visible mRNA band as protein evidence rather than evidence of transcription.

Recent exam appearances

May 2019Paper2 ["HL"] · TZ21(d)[ 1 ]D1.2.4—Transcription for gene expression
May 2017Paper2 ["HL"] · TZ21(f)(iii)[ 2 ]D1.2.4—Transcription for gene expression
May 2017Paper2 ["HL"] · TZ21(f)(ii)[ 2 ]D1.2.4—Transcription for gene expression
May 2017Paper2 ["HL"] · TZ11(e)[ 2 ]D1.2.4—Transcription for gene expression

Coverage 2017–2019 · 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.

Selective Transcription Controls Which Proteins a Cell Can Make

Gene expression is the use of gene information to produce a functional RNA or polypeptide. Transcription is a major control point because an untranslated gene cannot supply mRNA to ribosomes.

Transcription state Immediate consequence Possible protein outcome
gene active mRNA is produced translation can occur
gene inactive little or no mRNA is produced little or no corresponding polypeptide is made

Most cells in one organism contain the same genome, but different cell types transcribe different gene sets. Their different protein mixtures produce different structures and functions.

Turning transcription on permits expression; it does not guarantee a fixed protein amount. RNA processing, translation and protein breakdown can also be regulated.

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 for gene expression

[ 2 ]

The scientists concluded that auxin activates the transcription of the GH 3 gene. Using the information on the auxin concentration in the stem base in the graph on page 4 and the Northern blot, evaluate whether this conclusion is supported.

Concept essentials

  • mRNA presence indicates that a gene has been transcribed.
  • Different cells can express different genes from the same genome.
  • Gel bands and expression graphs can compare transcription across tissues or treatments.
  • Gene expression evidence must be linked to the named gene and condition.