Answer key: Codon Chart Activity
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Goal: Read the genetic code from a codon chart, find every codon for an amino acid, and explain why 64 codons code for only 20 amino acids.
Completed results table
| mRNA codon | Amino acid (three-letter) | One-letter code |
|---|---|---|
| UGG | Trp | W |
| AUG | Met | M |
| GCA | Ala | A |
| UUC | Phe | F |
| CCU | Pro | P |
| GAA | Glu | E |
| UAG | Stop | none (stop) |
Answers
How many codons code for leucine (Leu)? How many code for methionine (Met) and for tryptophan (Trp)?
Leucine has 6 codons: UUA, UUG, CUU, CUC, CUA and CUG. Methionine (AUG) and tryptophan (UGG) have just 1 each.
Translate the mRNA 5′-AUG GCA UUC UAG-3′ into amino acids.
AUG = Met (the start codon), GCA = Ala and UUC = Phe. UAG is a stop codon, so the chain ends there: Met-Ala-Phe, 3 amino acids.
There are 64 codons but only 20 amino acids. How many codons code for amino acids, and what do the others do?
61 codons code for amino acids and 3 (UAA, UAG and UGA) are stop signals. Most amino acids have more than one codon, so the code is redundant (degenerate).
Which three amino acids have six codons each?
Leucine, serine and arginine (Leu, Ser and Arg).
A DNA template strand reads 3′-TAC-5′. Which mRNA codon and amino acid does it give, and why must you transcribe it before using the chart?
The mRNA codon is AUG (each base pairs with its complement, with U opposite A), which codes for methionine, the start codon. The chart lists mRNA codons, so looking up TAC directly gives the wrong answer.
Challenge: a mutation changes the codon UGG to UGA. What happens to the protein?
UGG codes for tryptophan, but UGA is a stop codon, so translation ends early. This nonsense mutation gives a shortened protein that usually does not work.