The process of building proteins relies on instructions encoded within our genes. These instructions use a sequence of nucleotides to specify which components, known as amino acids, should be linked together. Amino acids are the building blocks of proteins, and each one helps determine a protein’s final structure and function. One of these is tryptophan, an amino acid that must be obtained through diet.
The genetic information in DNA is first transcribed into a messenger RNA (mRNA) molecule, which carries the instructions from the nucleus to the cell’s protein-building machinery. This code is read in three-letter “words” called codons, with each codon corresponding to a particular amino acid. This system ensures the twenty different amino acids are assembled in the correct order.
Identifying the Tryptophan Codon
The specific messenger RNA (mRNA) codon that directs the incorporation of tryptophan into a protein chain is UGG. In this triplet, U stands for uracil and G represents guanine, two of the four nucleotide bases found in RNA. This three-letter sequence is the universal signal for tryptophan in nearly all forms of life, highlighting a shared molecular language across species.
This genetic instruction originates from the cell’s DNA. During transcription, the DNA sequence is used as a template to create the mRNA molecule. The corresponding code for tryptophan on the DNA template strand is ACC, where adenine (A) pairs with uracil (U) and cytosine (C) pairs with guanine (G). This process ensures the UGG codon is accurately placed into the mRNA sequence.
The UGG codon exclusively signals for the addition of tryptophan. Any other three-letter combination will either code for a different amino acid or signal for the protein chain to stop growing. This precision allows for the assembly of complex proteins.
A Unique Codon Among Amino Acids
The genetic code possesses a feature known as degeneracy, or redundancy, meaning that most amino acids are specified by more than one codon. For instance, the amino acid Leucine is encoded by six different codons, while Arginine also has six. This redundancy provides a buffer against some mutations, as a change in the DNA sequence might not necessarily result in a different amino acid being added to the protein.
Tryptophan stands out because it lacks this redundancy. It is one of only two amino acids—the other being Methionine—that is encoded by a single codon. While Methionine is coded by AUG, tryptophan is exclusively coded by UGG, meaning any mutation to this codon will alter the instruction.
The evolutionary reason for tryptophan’s single codon is a subject of scientific discussion. One theory relates to its relative rarity and the high energy cost for an organism to synthesize it. Because tryptophan is the least abundant amino acid found in proteins, the selective pressure to have multiple codons may have been lower.
Role During Protein Synthesis
The translation of the genetic code into a protein occurs within a cellular machine called the ribosome. As the ribosome moves along the mRNA strand, it reads each codon sequentially. When the UGG codon enters the ribosome’s reading frame, it acts as a specific signal, pausing the machinery until the correct molecule arrives.
This is where a specialized molecule called transfer RNA (tRNA) comes into play. Each type of amino acid has a corresponding set of tRNA molecules responsible for carrying it. The tRNA molecule that recognizes the UGG codon for tryptophan has a complementary three-letter sequence on one of its loops, known as an anticodon. The anticodon for the tryptophan-carrying tRNA is ACC, which pairs precisely with the UGG codon on the mRNA.
This specific binding ensures that tryptophan is the only amino acid added to the growing protein chain at that position. Once the tRNA delivers its tryptophan cargo, the ribosome catalyzes the formation of a peptide bond, linking it to the preceding amino acid. The ribosome then advances to the next codon, continuing the process until the protein is assembled.
Consequences of Mutation
Given that UGG is the sole codon for tryptophan, any change to this sequence through mutation has significant consequences. These alterations, known as point mutations, can disrupt the protein-building process because there is no backup codon.
One of the most severe outcomes occurs if the UGG codon mutates to UGA. The UGA codon is one of three “stop” codons, which signal the ribosome to terminate protein synthesis. This change, a nonsense mutation, causes the protein chain to be cut short prematurely, and the resulting truncated protein is almost always non-functional.
Alternatively, a mutation could change the UGG codon into one that specifies a different amino acid, a missense mutation. For example, if the final guanine is replaced with a uracil or cytosine, the codon becomes UGU or UGC. Both of these codons specify the amino acid Cysteine, and the substitution can drastically alter the protein’s structure and function.