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

Protein Synthesis – Key Concepts (Cambridge IGCSE/A‑Level 9700)

1. DNA strands involved in transcription

Each gene has two complementary DNA strands. Only one of them is read by RNA polymerase to make messenger RNA (mRNA).

  • Transcribed (template) strand – read by RNA polymerase in the 3′→5′ direction; the resulting mRNA is complementary to this strand.

  • Non‑transcribed (coding) strand – runs 5′→3′ in the same orientation as the mRNA; its sequence is identical to the mRNA (with thymine (T) replaced by uracil (U)). This strand is not used as a template.

2. Comparison of the two strands

FeatureTranscribed (template) strandNon‑transcribed (coding) strand
Direction read by RNA polymerase3′→5′5′→3′ (same as mRNA)
Base sequence of resulting mRNAComplementary (A↔U, C↔G)Identical (T→U)
Role in transcriptionTemplate for synthesisNot used as template

3. The transcription process – step by step

  1. Promoter recognition

    • RNA polymerase II (core enzyme) together with general transcription factors binds to the promoter region.

    • Typical promoter elements: TATA box (~25‑30 bp upstream of the transcription start site) and a downstream Inr (initiator) sequence.

  2. Initiation

    • The enzyme locally unwinds the DNA, exposing the template strand.

    • RNA polymerase reads the template strand 3′→5′ and synthesises RNA 5′→3′, adding ribonucleoside‑triphosphates (NTPs).

    • Mg²⁺ ions and the energy released from NTP hydrolysis are required for each nucleotide addition.

  3. Elongation

    • RNA polymerase moves along the template strand, elongating the nascent mRNA.

    • The growing mRNA remains base‑paired with the DNA template until a termination signal is reached.

  4. Termination

    • In eukaryotes a poly‑adenylation signal (AAUAAA) and downstream cleavage site trigger release of the pre‑mRNA.

    • In prokaryotes termination may be rho‑dependent (rho factor binds the RNA) or rho‑independent (formation of a GC‑rich hairpin followed by a U‑rich tract).

  5. Post‑transcriptional processing (pre‑mRNA → mature mRNA)

    • Capping – addition of a 7‑methylguanosine cap to the 5′ end protects the mRNA and assists ribosome binding.

    • Poly‑A tail – a stretch of ~200 adenine nucleotides is added to the 3′ end, enhancing stability and export.

    • Splicing – introns are removed and exons joined by the spliceosome.

  6. Export to the cytoplasm

    • Mature mRNA passes through nuclear pores and becomes available for translation.

4. Translation – converting mRNA into a protein

  1. Ribosome structure

    • Two subunits: a small (40S in eukaryotes) and a large (60S) subunit that assemble on the mRNA.

    • The small subunit recognises the 5′‑cap and scans for the start codon (AUG).

  2. Key players

    • mRNA – provides the codon sequence.

    • tRNA – each carries a specific amino acid and an anticodon that pairs with a codon on the mRNA.

    • Ribosomal RNA (rRNA) – catalyses peptide‑bond formation.

  3. Stages of translation

    1. Initiation

      • eIFs (eukaryotic initiation factors) help the small ribosomal subunit bind the 5′‑cap and locate the start codon.

      • Met‑tRNAiMet (initiator tRNA) pairs with the AUG codon in the P‑site.

      • The large subunit joins, forming a complete ribosome.

    2. Elongation

      • Amino‑acyl‑tRNA matching the next codon enters the A‑site.

      • Peptide bond forms between the nascent chain (in the P‑site) and the new amino acid (in the A‑site).

      • The ribosome translocates 3′→5′ along the mRNA; the tRNA in the P‑site moves to the E‑site and exits, while the peptidyl‑tRNA moves to the P‑site.

    3. Termination

      • When a stop codon (UAA, UAG or UGA) reaches the A‑site, release factors (eRF1/eRF3 in eukaryotes) bind.

      • The polypeptide is released, and the ribosomal subunits dissociate.

  4. Result

    • A linear polypeptide chain that will fold into a functional protein, possibly undergoing further post‑translational modifications.

5. Gene‑mutation box (Topic 6.2)

Mutation typeDefinitionTypical effect on proteinExample
SubstitutionOne base is replaced by another.May change a single amino‑acid (missense), create a stop codon (nonsense), or be silent.Sickle‑cell disease – A→T substitution in the β‑globin gene changes Glu → Val.
DeletionOne or more bases are lost.Often causes a frameshift, altering the downstream reading frame.ΔF508 cystic fibrosis mutation – deletion of three nucleotides (phenylalanine) in the CFTR gene.
InsertionOne or more extra bases are added.Usually produces a frameshift, leading to a non‑functional protein.Huntington’s disease – expansion of CAG repeats (glutamine) in the HTT gene.

6. Visual summary (suggested diagram)

DNA double helix showing the transcribed (template) strand being read by RNA polymerase, the non‑transcribed (coding) strand, the nascent mRNA, and a ribosome translating the mRNA into a polypeptide.

7. Quick recap

  • The template (transcribed) strand is read 3′→5′ by RNA polymerase; the mRNA produced is complementary.

  • The coding (non‑transcribed) strand runs 5′→3′, matches the mRNA sequence (T→U), and is not a template.

  • Promoter elements (e.g., TATA box) position RNA polymerase; termination can be rho‑dependent or involve a hairpin.

  • Pre‑mRNA processing: 5′‑cap, poly‑A tail, splicing.

  • Translation uses ribosomes, tRNA, and three stages (initiation, elongation, termination) to build a polypeptide.

  • Mutations (substitution, deletion, insertion) can alter the coding sequence and affect protein structure/function.