state that the strand of a DNA molecule that is used in transcription is called the transcribed or template strand and that the other strand is called the non-transcribed strand

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Protein Synthesis Notes

Protein Synthesis – Key Concepts

1. DNA strands involved in transcription

During transcription, only one of the two strands of the DNA double helix serves as the template for RNA synthesis.

  • Transcribed (template) strand – the strand that is read by RNA polymerase to produce a complementary messenger RNA (mRNA) molecule.

  • Non‑transcribed strand – also called the coding strand; it has the same base sequence as the mRNA (except that thymine (T) is replaced by uracil (U) in RNA) and is not used as a template.

2. Relationship between the strands

FeatureTranscribed (Template) StrandNon‑transcribed (Coding) Strand
Direction of reading by RNA polymerase3' → 5'5' → 3' (same as mRNA)
Base sequence of resulting mRNAComplementary (A↔U, C↔G)Identical (except T→U)
Role in transcriptionTemplate for synthesisNot used as template

3. Transcription process – step by step

  1. RNA polymerase binds to the promoter region on the DNA.

  2. The enzyme unwinds a short segment of the double helix, exposing the transcribed (template) strand.

  3. RNA polymerase moves along the template strand in the 3' → 5' direction, adding ribonucleotides that are complementary to the DNA bases.

  4. Elongation continues until a termination signal is reached, producing a pre‑mRNA molecule.

  5. Pre‑mRNA undergoes processing (capping, poly‑A tail addition, splicing) to become mature mRNA ready for translation.

4. Visual summary

Suggested diagram: DNA double helix showing the transcribed (template) strand being read by RNA polymerase and the resulting mRNA strand aligned with the non‑transcribed (coding) strand.

5. Quick recap

Remember:

  • The transcribed/template strand is the strand that RNA polymerase reads to synthesize mRNA.

  • The non‑transcribed (coding) strand has the same sequence as the mRNA (with T replaced by U) but is not used as a template.