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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
Practice this objective

Coverage 2021–2023 · Updated 16 Jul 2026

Nucleic-Acid Synthesis Has a Defined Direction

HL only

Transcription and translation both proceed with respect to the mRNA 5′→3′ direction.

RNA polymerase reads the DNA template 3′→5′ and adds RNA nucleotides to the transcript's 3′ end, so RNA is synthesized 5′→3′.

The ribosome moves along mRNA from its 5′ end toward its 3′ end, reading successive codons in that direction while the polypeptide elongates.

Label both ends before interpreting a diagram: a template shown 3′→5′ supports a transcript drawn antiparallel 5′→3′, and the ribosome follows the transcript toward 3′.

‘Left to right’ is meaningless without strand labels. Transcription direction refers to new RNA synthesis; translation direction refers to ribosome movement along mRNA.

Directionality exam focus

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: RNA synthesis proceeds in the 5 prime to 3 prime direction.

Watch for

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

Representative question

Question 1

[Maximum number: 1]

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

HL Protein Synthesis Details

HL only

RNA polymerase reads template DNA 3' to 5' and synthesizes RNA 5' to 3'; ribosomes translate mRNA codons in the 5' to 3' direction. Promoters mark transcription start regions and orientation; transcription factors help RNA polymerase bind and initiate in eukaryotes. Non-coding DNA does not code for polypeptide amino acid sequences and includes introns, regulatory sequences, telomeres, rRNA genes, and tRNA genes. Eukaryotic pre-mRNA is modified before export and translation by adding a 5' cap and poly-A tail and removing introns by splicing. Alternative splicing joins different exon combinations from one pre-mRNA, so one gene can produce multiple protein variants in different cells or stages. Translation initiation assembles ribosomal subunits at the start codon AUG; initiator tRNA enters the P site and A, P, and E sites organize tRNA movement. Newly made polypeptides may be folded, cleaved, or chemically modified; preproinsulin processing to active insulin is a key example. Proteasomes degrade tagged, damaged, or unneeded proteins; amino acid recycling supports new protein synthesis and proteome quality control.

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.