Summary
AUG begins translation and codes methionine; UAA, UAG and UGA end it.
In depth
AUG sets the reading frame; in bacteria a Shine-Dalgarno sequence aligns the ribosome, while in eukaryotes the ribosome scans from the cap. Stop codons are recognised by proteins (release factors), not tRNAs.
Readthrough of stop codons can be exploited by some drugs.
The sequence around the start codon (Kozak sequence in eukaryotes, GCCRCCAUGG) affects start efficiency, and upstream open reading frames can control it. Some viruses use internal ribosome entry sites, structures that allow the ribosome to begin without scanning from the cap.
Stop codons can be misread: stop codon read-through occurs by suppressor tRNAs and in selenoprotein synthesis, where UGA is recoded as selenocysteine when a special structure is present. Drugs such as gentamicin can promote read-through in some nonsense mutations.
Remember: UAA, UAG and UGA have no tRNAs; release factors read them.
Frequently asked questions
What is Start and Stop Codons?
AUG begins translation and codes methionine; UAA, UAG and UGA end it.
Which area of biology covers Start and Stop Codons, and how advanced is it?
Start and Stop Codons is one of the 52 topics in Molecular Biology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Codon, Translation, Nonsense-Mediated Decay.
Can you give an example of Start and Stop Codons?
Selenocysteine is encoded by UGA in special contexts.
What should I learn before Start and Stop Codons?
Work through the Beginner topics in Molecular Biology first, such as Central Dogma, DNA Replication, Base Pairing, then return to Start and Stop Codons. Intermediate topics assume the Beginner vocabulary.
What topics are related to Start and Stop Codons?
The closest topics to Start and Stop Codons are Codon, Translation, Nonsense-Mediated Decay, Translational Control. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Start and Stop Codons?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Start and Stop Codons. To practise the whole of Molecular Biology, use the Molecular Biology quiz.
How is Start and Stop Codons different from Codon?
Start and Stop Codons: AUG begins translation and codes methionine; UAA, UAG and UGA end it. Codon: A triplet of mRNA bases that codes for one amino acid or a stop signal. Read both side by side and note what each one explains.
How should I study Start and Stop Codons step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Codon and explain the difference aloud. Finally, take the Molecular Biology quiz and come back to this page for any question you miss.