Step by Step
64
64 possible codons from 4 bases
With 4 bases (A, U, G, C) and 3-letter codons, there are 4³ = 64 possible combinations. 61 encode amino acids. 3 are stop codons. Only 20 amino acids exist — so most amino acids have multiple codons (the code is degenerate).
Memory trick: 4³ = 64. 61 amino acid codons + 3 stop codons = 64 total.
AUG
AUG = Start codon (Methionine)
Every protein synthesis begins with AUG — the start codon. It codes for methionine (Met). In prokaryotes, the first amino acid is formyl-methionine (fMet). The ribosome assembles at AUG and begins reading the mRNA.
Memory trick: AUG = "A UniGun starts the race." Or: AUG = Always U Go (start).
Stop
UAA, UAG, UGA = Stop codons
Three codons signal the end of translation — no tRNA recognizes them, so a release factor binds and the ribosome disassembles. They are UAA, UAG, and UGA.
Memory trick: UAA = "U Are Away." UAG = "U Are Gone." UGA = "U Go Away." All three mean STOP.
≈
The code is degenerate (redundant)
Most amino acids are coded by more than one codon. For example, leucine has 6 codons. This redundancy protects against silent mutations — a base change in the third position of a codon often still codes for the same amino acid.
Applied Walkthrough
1
An mRNA reads: 5'-AUG-UUU-GGC-UAA-3'. How many amino acids does this encode?
2
AUG = start (Met). UUU = Phenylalanine. GGC = Glycine. UAA = STOP.
3
The protein is 3 amino acids long: Met-Phe-Gly. UAA terminates translation — it is not incorporated.
4
Note: in some contexts the initial Met is cleaved after translation, leaving a 2-amino acid protein.
Exam Application
Exams test whether you know the start codon (AUG), can identify stop codons (UAA, UAG, UGA), and understand degeneracy. Questions often give an mRNA sequence and ask how many amino acids are produced — remember to count from AUG and stop before the stop codon.
⚠ Common Trap
Students count the stop codon as an amino acid — it's not. Also, "degenerate" does NOT mean the code is broken or imprecise — it means redundant (multiple codons for one amino acid). Don't confuse degenerate with ambiguous: the code is NOT ambiguous — each codon specifies exactly one amino acid.
✓ Quick Self-Check
1. How many possible codons are there, and how many encode amino acids?
64 total codons; 61 encode amino acids; 3 are stop codons.
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2. What is the start codon and what amino acid does it encode?
AUG — encodes methionine (Met).
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3. What are the three stop codons?
UAA, UAG, and UGA.
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4. What does "degenerate" mean in the context of the genetic code?
Multiple codons can encode the same amino acid — the code is redundant, not ambiguous.
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5. The mRNA 5'-AUG-CCG-UAG-3' encodes how many amino acids?
2 amino acids: Met + Proline. UAG is a stop codon and is not incorporated.
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