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D1.2.2—Hydrogen bonding in transcription

Hydrogen bonding allows RNA nucleotides to align with the DNA template during transcription through specific complementary base pairing with exposed bases.

Syllabus
First assessment 2025
Objective
D1.2.2
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper1
Typical marks1

Common command terms

  • Determine

Scoring notes

Common mistake
Writing thymine in an mRNA sequence instead of uracil.

Recent exam appearances

May 2024Paper1 ["HL"] · TZ128[ 1 ]D1.2.2—Hydrogen bonding in transcription
May 2017Paper1 ["HL"] · TZ19[ 1 ]D1.2.2—Hydrogen bonding in transcription
November 2014Paper1 ["HL"] · TZ030[ 1 ]D1.2.2—Hydrogen bonding in transcription
May 2013Paper1 ["HL"] · TZ17[ 1 ]D1.2.2—Hydrogen bonding in transcription
November 2012Paper1 ["HL"] · TZ027[ 1 ]D1.2.2—Hydrogen bonding in transcription

Coverage 2012–2024 · Updated 16 Jul 2026

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.

Transcription Opens DNA Locally and Leaves Its Sequence Unchanged

Hydrogen bonds between complementary bases can break and re-form. This lets a short DNA region open temporarily without breaking the covalent sugar–phosphate backbones.

After the mRNA leaves, the original complementary DNA strands pair again. The gene's base sequence is conserved and can be transcribed repeatedly.

Transcription copies one gene region into RNA; it does not duplicate the whole DNA molecule and does not consume the template strand.

DNA opens locally for mRNA synthesis, the mRNA leaves and the original DNA double helix reforms.

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.

Hydrogen bonding in transcription

[ 1 ]

The sequence of bases on a short section of the antisense strand of a gene undergoing transcription is shown:

5′ CATG 3′5^{\prime} \text { CATG } 3^{\prime}

What is the sequence of bases on the resulting mRNA?

Concept essentials

  • RNA uses uracil instead of thymine during base pairing.
  • The mRNA strand is complementary to the antisense DNA template.
  • The sense DNA strand matches mRNA except for thymine and uracil.
  • Hydrogen bonds align RNA nucleotides before covalent bonds form.