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
- SL
DNA Stays Stable While Genes Are Read
A single DNA strand can act repeatedly as a transcription template without its base sequence changing.
RNA polymerase reads the template and joins separate RNA nucleotides; it does not consume or replace the DNA bases. After the transcription bubble passes, the DNA strands pair again.
DNA opens locally → RNA copy is synthesized → RNA leaves → original DNA sequence remains conserved and available for later transcription.
A non-dividing neuron may transcribe the same essential gene many times while conserving that gene's DNA sequence throughout the life of the cell.
Stable does not mean inaccessible: local opening permits transcription. The RNA transcript can change or degrade while the DNA template sequence remains unchanged.
Core Protein Synthesis
- Transcription: RNA polymerase builds complementary mRNA from the DNA template; A pairs with U and C with G.
- Translation: ribosomes read mRNA codons 5′ to 3′ while tRNA anticodons deliver specific amino acids for peptide-bond formation.
- Genetic code: codons are triplets; the code is degenerate and almost universal, with start and stop signals. Use mRNA—not DNA—when reading a code table.
- Information flow: codon order determines amino-acid sequence, which determines protein folding and function.
- Expression and variation: cells regulate which genes are transcribed. A mutation may change a codon, primary structure and phenotype, as in sickle-cell haemoglobin.
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.