← Gene expression

Reading and completing transcription/translation diagrams

The reliable Achieved-level marks every year: correctly filling in a partially-completed DNA/mRNA/tRNA diagram using the base-pairing and codon-table rules.

The checklist for completing any given diagram

  • Identify which strand is the TEMPLATE strand (the one actually being read) — mRNA is built by pairing to THIS strand, not the other DNA strand.
  • Apply base-pairing rules exactly: DNA template A pairs with mRNA U; DNA template T pairs with mRNA A; DNA template C pairs with mRNA G; DNA template G pairs with mRNA C.
  • Once mRNA is written, group it into CODONS (three bases at a time), starting from the correct reading frame (usually signalled by a start codon, AUG).
  • tRNA anticodons are complementary to the mRNA codons using the SAME base-pairing rules — write each one directly below its matching codon.
  • Use the codon table PROVIDED in the resource booklet to convert each codon into its amino acid — do not rely on memorising codon-to-amino-acid mappings, since the table is always supplied.
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Sugars, phosphates and strand direction

Some diagrams also ask you to label the sugar-phosphate backbone and the 5' / 3' ends of each strand. Remember DNA's two strands run ANTIPARALLEL (opposite directions) — the 5' end of one strand lines up against the 3' end of the other. Getting this orientation backwards is a common diagram-labelling error. Worked example Completing an mRNA and amino-acid sequence from a DNA template DNA template: TAC GGA TTC. Complete the mRNA and amino acid sequence (AUG=met, CCU=pro, AAG=lys — reference table given). Pair each DNA base to its RNA complement: T→A, A→U, C→G, G→C, G→C, A→U, T→A, T→A, C→G. mRNA: AUG CCU AAG. Look up each codon in the provided table: AUG = methionine, CCU = proline, AAG = lysine. ✓ met–pro–lys. Working base-by-base rather than trying to jump straight to the amino acids avoids pairing errors.

A fast check before moving past a diagram

  • Have I identified the TEMPLATE strand correctly before pairing bases, not the coding strand?
  • Did I group the completed mRNA into codons from the correct reading frame before using the codon table?

Practice for this standard covers the whole topic — flashcards and quiz.