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D1.2.14 (HL)—Non-coding sequences in DNA

Non-coding DNA includes sequences that regulate genes, protect chromosome ends or code for functional RNA rather than polypeptides in genomes.

Syllabus
First assessment 2025
Objective
D1.2.14
Level
HL

Exam analysis

Chance of appearing10%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1

Common command terms

  • State

Scoring notes

Common mistake
Equating non-coding DNA with having no function.

Recent exam appearances

November 2024Paper1 ["HL"] · TZ027[ 1 ]D1.2.14 (HL)—Non-coding sequences in DNA
November 2022Paper2 ["HL"] · TZ04(c)(ii)[ 1 ]D1.2.14 (HL)—Non-coding sequences in DNA
May 2021Paper1 ["HL"] · TZ126[ 1 ]D1.2.14 (HL)—Non-coding sequences in DNA
May 2019Paper2 ["HL"] · TZ22(a)(iii)[ 2 ]D1.2.14 (HL)—Non-coding sequences in DNA
May 2019Paper1 ["HL"] · TZ214[ 1 ]D1.2.14 (HL)—Non-coding sequences in DNA

Coverage 2013–2024 · Updated 16 Jul 2026

Non-Coding DNA Includes Several Functional Categories

HL only

Non-coding DNA is DNA that is not translated into a polypeptide sequence. Non-coding does not mean untranscribed, unused or without function.

Important non-coding categories include:

  • Regulatory sequences such as promoters and enhancers that affect transcription.
  • Introns that are transcribed into pre-mRNA and removed during splicing.
  • Genes for functional RNAs, including rRNA and tRNA, whose products are not translated.
  • Structural regions such as telomeres and centromeric DNA.
  • Repeated sequences, some functional and some with no known function.

An exon can include untranslated regions, so exon is not a synonym for protein-coding sequence. Likewise, an intron can influence regulation or splicing even though it is removed from mature mRNA.

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.

Non-coding sequences in DNA

HL only

[ 2 ]

DNA has regions that do not code for proteins. State two functions of these regions.
1.
2.

Concept essentials

  • Non-coding DNA can regulate transcription.
  • Telomeres protect the ends of chromosomes.
  • Some DNA sequences code for functional RNAs such as tRNA or rRNA.
  • Introns are removed from eukaryotic RNA before mature mRNA is translated.