Proteins govern the body’s intricate processes, and instructions for building them are stored in DNA. These instructions are carried out by messenger RNA (mRNA), which acts as a working copy of the genetic blueprint. To translate this linear genetic information into a protein’s structure, the cell reads the mRNA in small, discrete units. The fundamental reading unit of this information is called the codon, which ensures the correct amino acid sequence is assembled.
The Triplet Structure and Primary Function
A codon is defined as a sequence of three nucleotides found on the messenger RNA molecule. These nucleotides are the building blocks of RNA—Adenine (A), Uracil (U), Cytosine (C), and Guanine (G). A codon is always read as a triplet, such as AUG or GGC. This three-base structure is the minimum necessary arrangement to encode the 20 standard amino acids used in all proteins.
If the genetic code used only two nucleotides per unit, only 16 unique combinations would be possible (4² = 16), which is insufficient to specify the 20 amino acids plus the necessary stop signal for protein synthesis. Using a triplet generates 64 distinct combinations (4³ = 64), providing more than enough capacity to code for all 20 amino acids. This ensures that each codon specifies one particular amino acid or a signal to stop construction. The reading frame is fixed and read in a continuous, non-overlapping fashion from the beginning of the gene to the end.
Decoding the Blueprint: The Genetic Code
The complete set of 64 possible codons and the amino acids or signals they represent is known as the genetic code. This code operates like a universal lookup chart, where each three-base sequence is precisely matched to an amino acid. Sixty-one of the 64 codons are referred to as “sense” codons because they specify one of the 20 amino acids.
The most notable feature of the genetic code is its redundancy, sometimes called degeneracy, meaning that multiple codons can specify the same amino acid. For instance, Leucine is encoded by six different codons, and many other amino acids are specified by two or four. This redundancy provides a buffer against potential errors during DNA replication or transcription. A change in a single nucleotide may still result in the same amino acid being incorporated into the protein, minimizing the impact of some errors. The code is unambiguous, meaning that any single codon will only specify one particular amino acid.
Signaling the Process: Start and Stop Codons
Certain codons have specialized regulatory functions that act as punctuation marks for protein synthesis. The most common start codon is AUG, which signals the beginning of the protein chain and codes for the amino acid Methionine. The presence of AUG establishes the correct reading frame for the ribosome, ensuring that all subsequent codons are read in the proper three-base groupings.
At the end of the gene sequence, three specific codons—UAA, UAG, and UGA—act as stop codons, or termination signals. These three codons do not code for any amino acid; instead, they signal the ribosome to halt translation. When the ribosome encounters a stop codon, specialized release factors bind to the site, causing the newly formed protein chain to detach and completing the synthesis.
When the Code Changes: Codons and Genetic Mutations
An alteration in a codon’s nucleotide sequence constitutes a genetic mutation. A point mutation involves the substitution of a single nucleotide within a codon, which can lead to one of three outcomes. A silent mutation occurs when the change results in a different codon that still specifies the original amino acid, an outcome made possible by the genetic code’s redundancy.
A missense mutation replaces the original amino acid with a different one, potentially altering the protein’s structure and function, such as the single base change that causes sickle cell anemia. A nonsense mutation is a severe substitution where a sense codon changes into one of the three stop codons, leading to premature termination of the protein chain. Mutations involving the insertion or deletion of one or two nucleotides are known as frameshift mutations. Since the code is read in triplets, adding or removing a base shifts the entire reading frame for every subsequent codon, drastically changing the sequence of all downstream amino acids and almost always resulting in a nonfunctional protein.