Lesson 6 of 6 / DNA and gene expression
Read codons to build a polypeptide
How does the message become an amino-acid order?
In this lesson: Describe translation using codons, tRNA, ribosomes and peptide-bond formation.
About 6 min
The key ideaA ribosome reads mRNA codons 5' to 3'. Matching tRNAs deliver amino acids, which are joined in order until a stop codon ends translation.
Explore the idea
Read a message in its frame
AUG establishes the start in this model and the appropriate tRNA supplies methionine.
Original short-message model. Real polypeptides usually contain many more amino acids. A codon table is read using mRNA codons, not template DNA or anticodons.
Explanation
Translation begins when a ribosome assembles on the mRNA at an appropriate start codon, normally AUG in the standard model. A tRNA with the matching anticodon brings methionine. Establishing the reading frame determines how later bases are grouped into triplets.
Successive tRNAs pair their anticodons with exposed mRNA codons. The ribosome positions the amino acids and catalyses peptide-bond formation. It moves along the message 5' to 3', allowing the chain to extend and empty tRNAs to leave for reuse.
A stop codon does not encode another amino acid. Release factors bring translation to an end and the polypeptide is released. The chain then folds and may undergo further processing or join other subunits before becoming fully functional.
The genetic code is degenerate: several codons can specify the same amino acid. This helps explain why some base substitutions do not change the polypeptide sequence. A codon table is read using mRNA codons, not DNA template triplets or tRNA anticodons.
Protein synthesis transfers information; it does not consume the gene as building material. Amino acids provide the material for the chain, while energy is needed for translation and accurate delivery.
Step by step
- 1
Set the reading frame
Locate the start and group the message into codons.
- 2
Match and join
tRNA anticodons pair with codons and peptide bonds join delivered amino acids.
- 3
Stop and fold
A stop codon releases the chain; the polypeptide can then acquire its functional shape.
Worked example
Translate a supplied code
An mRNA segment is 5'-AUG CCU UGA-3'. Given AUG = methionine, CCU = proline and UGA = stop, what chain is produced from this start?
One way to explain it
Methionine-proline, then termination. UGA is not a third amino acid.
Why this answer works
- Read the mRNA in the displayed 5'-to-3' direction.
- Use the supplied codon meanings.
- Treat stop as an instruction to release, not a building unit.
Is this true? "Each stop codon adds a special stop amino acid."
Stop codons signal termination; no stop amino acid is incorporated into the chain.