9477 / 2027

Lesson 6 of 6 / DNA replication and gene expression

Translate codons into a polypeptide

How does a triplet of RNA bases specify an amino acid?

I can explain ribosomes, tRNA, initiation, elongation and termination.

  • I can explain DNA structure and the roles and structures of mRNA, tRNA and rRNA.Syllabus

    Syllabus 9477, Core 2(a). Explain DNA structure and the roles and structures of mRNA, tRNA and rRNA.

  • I can explain transcription, eukaryotic RNA processing and translation.Syllabus

    Syllabus 9477, Core 2(c). Explain transcription, eukaryotic RNA processing and translation.

About 7 min

Make a guess. You are not marked.

A ribosome reaches a stop codon on the mRNA. Which tRNA binds there?

  1. None. A release factor binds instead.
  2. A tRNA carrying a special stop amino acid.
  3. The start tRNA, to begin the next protein.
Show the answer

None. A release factor binds instead.

Stop codons have no matching tRNA. A release factor frees the finished polypeptide.

The key idea

A ribosome reads mRNA codons 5-prime to 3-prime; matching charged tRNAs deliver amino acids for peptide-bond formation.

Explore the idea

Match a codon, then extend the chain

Growing polypeptideMet3'-UAC-5'tRNA5'-AUG-3'Ribosome reads this codonN end -> growing C end

Initiation establishes the reading frame at AUG. The initiator tRNA carries methionine and has the complementary, antiparallel anticodon 3-prime UAC 5-prime.

The diagram shows the message triplet at each step, not three separate mRNAs. It compresses tRNA entry, peptide transfer and translocation into one stage; ribosomal sites and accurate molecular shapes are omitted. Amino-acid charging and translation require energy.

Explanation

The genetic code relates mRNA base triplets, or codons, to amino acids or stop signals. It is degenerate because more than one codon can specify the same amino acid, but an individual codon has a defined meaning in the relevant code. Correct amino-acid attachment to tRNA is essential before it reaches the ribosome.

Initiation establishes the reading frame at a start codon, commonly AUG, with an initiator tRNA and ribosomal subunits. During elongation, a charged tRNA with a complementary anticodon binds at the appropriate site. Ribosomal rRNA catalyses peptide-bond formation, transferring the growing chain to the incoming amino acid.

The ribosome moves along the mRNA by one codon; empty tRNA leaves and another charged tRNA enters. Energy is required for accurate charging and translation steps. The polypeptide grows from its amino end towards its carboxyl end while the message is read 5-prime to 3-prime.

At a stop codon, a release factor promotes release of the completed polypeptide; a stop codon does not encode an extra stop amino acid. Multiple ribosomes can translate one mRNA as a polysome. The chain may then fold, be modified or join other subunits to become functional.

Step by step
  1. 1

    Set the reading frame

    Begin at the specified start codon.

  2. 2

    Use the mRNA code

    Read codons 5-prime to 3-prime in non-overlapping triplets.

  3. 3

    Stop correctly

    Exclude the stop signal from amino-acid counts.

Worked example

Work through the evidence

An mRNA coding region has 303 bases from its start codon through its stop codon. How many amino acids are specified?

One way to explain it

303/3 = 101 codons, including one stop codon, so 100 amino acids are specified.

Why this answer works
  • A start codon specifies an amino acid.
  • The terminal stop codon does not.

Watch out for this

A student says: "A stop codon is matched by a tRNA carrying a stop amino acid." What is wrong with this?

Show the answer

Termination uses a release factor; there is no stop amino acid.

Check your understanding

Which statement correctly relates codon and anticodon?

  1. A tRNA anticodon pairs complementarily with an mRNA codon.
  2. Both are triplets on DNA.
  3. They pair in the same 5-prime to 3-prime direction.
Show the answer

A tRNA anticodon pairs complementarily with an mRNA codon.

This connects the message to delivery of the corresponding amino acid.

Exam-style questions

Check your understanding

Which statements about translation are correct? 1: The ribosome moves along mRNA one codon at a time. 2: Each tRNA can carry any amino acid. 3: Peptide bonds form between amino acids. 4: A stop codon is read by a tRNA carrying no amino acid.

  1. 3 and 4 only
  2. 1, 3 and 4 only
  3. 1 and 3 only
  4. 1, 2 and 3 only
Show the answer

1 and 3 only

The ribosome moves codon by codon and peptide bonds join amino acids. Each tRNA carries one specific amino acid, and stop codons bind release factors.

Quick recall

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

How does a triplet of RNA bases specify an amino acid?

You can return to this lesson any time.