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Lesson 6 of 6 / DNA and gene expression

Read codons to build a polypeptide

How does the message become an amino-acid order?

I can describe translation using codons, tRNA, ribosomes and peptide-bond formation.

  • I can describe transcription, pre-mRNA processing and translation.Syllabus

    Syllabus 8876, Core 2(c). Describe transcription, pre-mRNA processing and translation.

About 6 min

Make a guess. You are not marked.

Which molecule actually decides the order of amino acids in a new polypeptide?

  1. The tRNA, which arrives in any order.
  2. The order of codons on the mRNA.
  3. The ribosome, which chooses amino acids.
Show the answer

The order of codons on the mRNA.

The ribosome reads codons in turn. tRNA anticodons match them and deliver the right amino acids.

The key idea

A 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

AUGCCUUGA5'3'Methionine

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. 1

    Set the reading frame

    Locate the start and group the message into codons.

  2. 2

    Match and join

    tRNA anticodons pair with codons and peptide bonds join delivered amino acids.

  3. 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.

Watch out for this

A student says: "Each stop codon adds a special stop amino acid." What is wrong with this?

Show the answer

Stop codons signal termination; no stop amino acid is incorporated into the chain.

Check your understanding

What directly determines the order in which amino acids are added?

  1. The order of mRNA codons read by the ribosome.
  2. The amino acids with the highest cytoplasmic concentration are added first.
  3. The sequence of tRNA molecules produced first in the cell, regardless of codons.
Show the answer

The order of mRNA codons read by the ribosome.

Codon order directs the amino-acid sequence.

Quick recall

Card 1 of 3. Answer in your head, then check.

How does the message become an amino-acid order?

You can return to this lesson any time.