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?
In this lesson: Explain ribosomes, tRNA, initiation, elongation and termination.
About 7 min
The key ideaA 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
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
Set the reading frame
Begin at the specified start codon.
- 2
Use the mRNA code
Read codons 5-prime to 3-prime in non-overlapping triplets.
- 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.
Is this true? "A stop codon is matched by a tRNA carrying a stop amino acid."
Termination uses a release factor; there is no stop amino acid.