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D1.2.3—DNA template stability

The DNA template remains chemically stable during transcription because its sequence is read repeatedly without being consumed, copied permanently or altered.

Syllabus
First assessment 2025
Objective
D1.2.3
Level
HL

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.

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.

Concept essentials

  • The DNA template is read during transcription but is not consumed.
  • Temporary hydrogen bonding allows copying without changing the DNA base sequence.
  • The sugar-phosphate backbone preserves the continuity of the template strand.
  • DNA can be reused for repeated transcription of the same gene.