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D1.2.12 (HL)—Directionality

Directionality in gene expression means RNA is synthesized and mRNA is translated in defined 5 prime to 3 prime directions during synthesis.

Syllabus
First assessment 2025
Objective
D1.2.12
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2023
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Scoring notes

Common mistake
Choosing a start codon without using the 5 prime to 3 prime direction shown in the diagram.

Recent exam appearances

November 2023Paper3 ["HL"] · TZ23(c)[ 1 ]D1.2.12 (HL)—Directionality
May 2021Paper1 ["HL"] · TZ227[ 1 ]D1.2.12 (HL)—Directionality

Coverage 2021–2023 · Updated 16 Jul 2026

Information Is Read in Opposite Strand Directions but Built 5′→3′

HL only
Process Molecule being read Reading direction Product growth or movement
transcription DNA template 3′→5′ RNA is synthesized 5′→3′
translation mRNA 5′→3′ ribosome moves toward the mRNA 3′ end; polypeptide grows N-terminus→C-terminus

Nucleotides are added to a free 3′ hydroxyl group, so every new nucleic-acid strand grows 5′→3′. The template must therefore be read antiparallel to the new strand.

A diagram arrow showing polymerase movement and an arrow showing new-strand growth refer to opposite strands; identify which molecule each arrow belongs to.

RNA polymerase reads a DNA template 3′ to 5′ while synthesizing RNA 5′ to 3′; a ribosome then moves along mRNA 5′ to 3′.

Control Protein Output at Several Checkpoints

HL only
Checkpoint What can change Consequence for protein output
transcription initiation promoter and transcription-factor activity amount of pre-mRNA made
RNA processing cap, tail and exon combination mRNA stability, export and encoded sequence
translation initiation successful AUG recognition and ribosome assembly whether and where translation begins
post-translational processing folding, cleavage, modification and assembly protein activity or location
proteasomal degradation ubiquitin tagging and breakdown rate protein lifetime and abundance

Across these checkpoints, sequence direction stays consistent: RNA is made 5′→3′, the ribosome reads mRNA 5′→3′ and the polypeptide grows from N-terminus to C-terminus.

Gene expression is therefore not a single on/off event. A cell controls which message is made, how it is processed, whether it is translated, how the product is activated and how long the protein remains.

Directionality exam focus

HL only

[ 1 ]

Using the diagram, identify, with a reason, whether X or Y is the start codon.

Concept essentials

  • RNA synthesis proceeds in the 5 prime to 3 prime direction.
  • The ribosome reads mRNA codons from 5 prime to 3 prime.
  • Start codon position is interpreted using mRNA direction.
  • Template and product strands have opposite orientations during transcription.