Understanding start and stop codons is crucial for grasping the fundamentals of protein synthesis — a core Step 1 topic linking molecular biology and clinical pathology. These codons signal where translation begins and ends, ensuring proper protein assembly.
Start Codon (AUG)
| Feature | Eukaryotes | Prokaryotes |
|---|---|---|
| Codon | AUG | AUG |
| Amino Acid | Methionine (Met) | N-formylmethionine (fMet) |
| Mnemonic | AUG in AUGurates protein synthesis | fMet = “formyl” start |
| Clinical Note | Methionine may be removed after translation is completed | fMet triggers neutrophil chemotaxis (important in infection and inflammation) |
Stop Codons (Termination Codons)
| Stop Codon | Mnemonic | Function |
|---|---|---|
| UGA | “U Go Away” | Signals ribosome release |
| UAA | “U Are Away” | Signals termination |
| UAG | “U Are Gone” | Signals termination |
| Recognized by | Release factors | Not tRNA |
Key Points to Remember
- Translation always starts at AUG in mRNA (Met/fMet).
- No tRNA binds stop codons — instead, release factors terminate translation.
- Mutations that alter start or stop codons can lead to truncated or nonfunctional proteins, contributing to disease.
- fMet’s chemotactic property links bacterial infection to inflammatory responses.
Learning Objective
By the end of this topic, the learner should be able to:
Identify the function and clinical relevance of start and stop codons in protein synthesis and differentiate their roles in prokaryotes and eukaryotes.
