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VCE Biology (Year 12) · Nucleic Acids and Proteins · 12m read

Translation — From mRNA to Protein

Ever wonder how your cells turn genetic instructions into the proteins that make you work? Translation is where that magic happens—at structures called ribosomes, your cell's protein factories read the mRNA code three letters at a time (called codons), and special molecules called tRNA bring the right amino acids to build your protein piece by piece. Understanding this process helps you see how a tiny change in your DNA can sometimes create big problems, and it's crucial for VCE biology and fields like medicine and genetic engineering.

Learning objective: Describe translation at the ribosome, including the role of tRNA and codons, and determine amino acid sequences from mRNA.

By the end of this lesson you can:

  • I use the codon table to translate mRNA into amino acids.
  • I identify start (AUG) and stop codons.
  • I describe the role of tRNA and its anticodon.

Translation is a factory assembly line. Ribosomes read mRNA codons; tRNA delivers matching amino acids; a polypeptide chain is assembled.

Codons, tRNA and the ribosome

mRNA is read in groups of three nucleotides called codons. There are 64 possible codons and 20 standard amino acids, so the code is redundant (most amino acids have multiple codons). AUG is the universal start codon — it codes for methionine and marks where translation begins. UAA, UAG and UGA are stop codons — they signal release of the polypeptide.

Transfer RNA (tRNA) carries amino acids to the ribosome. Each tRNA has an anticodon (complementary to a codon) and the matching amino acid attached at its 3' end. When an anticodon pairs with a codon in the ribosome's A site, the amino acid is added to the growing polypeptide.

The ribosome has three sites: E (exit), P (peptidyl), A (aminoacyl). Elongation cycles tRNAs through A → P → E as the polypeptide grows.

Translation essentials

  • Start codon: AUG — methionine — begins translation.
  • Stop codons: UAA, UAG, UGA — release polypeptide.
  • Codon length: 3 nucleotides.
  • Redundancy: Most amino acids have multiple codons.
  • tRNA role: Carries amino acid; anticodon pairs with codon.

Quick reference

| Component | Role |
|---|---|
| mRNA codon | 3 bases code an amino acid |
| tRNA anticodon | Matches codon |
| Ribosome | Site of translation |
| Start codon | AUG (methionine) |
| Stop codons | UAA, UAG, UGA |

Worked example (Medium)

Translate the mRNA 5'-AUGCCGUAA-3' into an amino acid sequence. (Standard codon chart: AUG = Met, CCG = Pro, UAA = Stop.)

  1. Split into codons. Read 5'→3' in triplets. AUG | CCG | UAA
  2. Translate each codon. Match against the standard code. Met – Pro – Stop
  3. Assemble polypeptide. Stop is not a residue. Met–Pro (2 amino acids)

Answer: Polypeptide: Met–Pro (translation stops at UAA).

Worked example (Hard)

An mRNA sequence reads 5'-AUGGCUUACUAGUAA-3'. Determine the amino acid sequence and identify any issues.

  1. Find start codon. AUG (methionine) at position 1 — translation begins here.
  2. Read codons in frame. AUG | GCU | UAC | UAG | UAA.
  3. Translate. AUG = Met, GCU = Ala, UAC = Tyr, UAG = STOP.
  4. Result. Polypeptide: Met-Ala-Tyr (3 amino acids). UAG is the stop codon; UAA after it is not read.

Answer: Met-Ala-Tyr. Translation terminates at the first stop codon (UAG). The polypeptide is 3 amino acids long.

Common mistakes

  • Including the stop codon as an amino acid in the polypeptide Stop codons (UAA, UAG, UGA) do not code for amino acids — they signal release. Fix: Stop codons trigger release factors that detach the polypeptide from the ribosome. Never write an amino acid for a stop codon.
  • Reading codons from the wrong starting point (wrong reading frame) A shift of just one nucleotide completely changes all downstream codons. Fix: Always start reading from the AUG start codon. Each subsequent codon is the next three nucleotides in sequence.
  • Confusing anticodon direction with codon direction The anticodon on tRNA is complementary AND antiparallel to the mRNA codon. Fix: If the mRNA codon is 5'-AUG-3', the tRNA anticodon is 3'-UAC-5'. Always check both complementarity and antiparallel orientation.

The genetic code — universal, redundant, and unambiguous

The genetic code has three remarkable properties that VCE Biology students must understand deeply.

Universal: Nearly all organisms — from bacteria to humans — use the same genetic code. AUG codes for methionine in *E. coli*, in yeast, and in humans. This universality is powerful evidence for common ancestry — all life inherited this code from a single ancestral cell. Minor exceptions exist in mitochondria and some protists, but these are rare variations on a fundamentally shared code.

Redundant (degenerate): Most amino acids are encoded by more than one codon. Leucine, for example, has six codons (UUA, UUG, CUU, CUC, CUA, CUG). This redundancy provides a buffer against mutation — many point mutations in the third position of a codon do not change the amino acid (silent/synonymous mutations). This is why the third position is sometimes called the "wobble" position.

Unambiguous: Each codon specifies exactly ONE amino acid (or stop signal). There is no ambiguity — GCU always means alanine, never anything else. This ensures faithful translation of the genetic message.

VCE application: Questions about mutation often require understanding redundancy. A mutation from GCU to GCC is silent (both = alanine). A mutation from GCU to GAU changes alanine to aspartate — a missense mutation that could alter protein function. The sickle cell mutation (GAG → GUG, glutamate → valine in haemoglobin) is a classic VCE example of how a single base change causes disease.

Practice questions

  1. The start codon in eukaryotic translation is:

    1. AUG
    2. UAA
    3. TAC
    4. GCA

    (2 marks)

  2. If a tRNA has anticodon 3'-UAC-5', which codon does it pair with on mRNA?

    1. 5'-AUG-3'
    2. 5'-UAC-3'
    3. 5'-GUA-3'
    4. 5'-AGC-3'

    (3 marks)

  3. Explain why a single-nucleotide substitution can have no effect on the protein produced.

    (3 marks)

Summary

  • Use the codon table to translate mRNA into amino acids.
  • Identify start (AUG) and stop codons.
  • Describe the role of tRNA and its anticodon.

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