Deoxyribonucleic acid (DNA) contains the instructions necessary for an organism’s development, functioning, and maintenance. Cellular machinery reads these instructions to produce proteins, which are fundamental to virtually all biological processes.
Understanding the Triplet
A codon is a fundamental unit of genetic information, a specific sequence of three nucleotides. This “triplet” nature provides enough unique combinations to specify the 20 different amino acids that form proteins.
The genetic alphabet is composed of four types of nucleotide bases: adenine (A), guanine (G), cytosine (C), and either thymine (T) in DNA or uracil (U) in RNA. These four bases, when arranged in groups of three, create 4 x 4 x 4 = 64 possible codon combinations.
While the original genetic instructions reside in DNA, codons are primarily found on messenger RNA (mRNA) molecules. Messenger RNA is a temporary copy of a segment of DNA, created during a process called transcription, and it carries the genetic message from the DNA to the cellular machinery responsible for protein synthesis.
How Codons Direct Protein Production
The genetic code refers to the rules by which genetic material is translated into proteins. Each mRNA triplet codon corresponds to a specific amino acid. This process of converting mRNA codons into an amino acid sequence is called translation.
During translation, ribosomes read the mRNA sequence. As a ribosome moves along the mRNA, it encounters each codon, recruiting the appropriate transfer RNA (tRNA) molecule. Each tRNA carries a specific amino acid and has an “anticodon” complementary to the mRNA codon. This matching ensures the correct amino acid is added to the growing protein chain, forming peptide bonds.
The reading frame’s integrity is important; the ribosome must read codons in groups of three without overlap or shift, ensuring accurate protein production. If the reading frame is disrupted, the entire downstream amino acid sequence will be incorrect, leading to a non-functional protein.
Special Signals in the Genetic Code
Certain codons serve as signals within the genetic code. The “start codon,” typically AUG, marks the precise point where protein synthesis should begin. This codon also codes for the amino acid methionine, meaning that most newly synthesized proteins initially begin with methionine.
Conversely, there are “stop codons”—UAA, UAG, and UGA—which do not code for any amino acid. Instead, these three codons act as termination signals, prompting the ribosome to release the newly synthesized protein chain and conclude the translation process.
The genetic code also exhibits “redundancy” or “degeneracy,” meaning multiple codons can specify the same amino acid. For example, leucine is encoded by six different codons. This redundancy provides a protective mechanism, as a single nucleotide change might still result in the same amino acid, minimizing mutation impact.
What Happens When Codons Change?
Alterations in the DNA sequence, known as mutations, can lead to changes in the mRNA codons, potentially affecting the resulting protein. When a codon changes but still codes for the same amino acid due to the degeneracy of the genetic code, it is termed a “silent mutation.” These mutations often have no observable effect on the protein’s function.
In contrast, a “missense mutation” occurs when a codon change results in the incorporation of a different amino acid into the protein sequence. The impact of a missense mutation can vary, ranging from negligible to severe, depending on the new amino acid’s properties and its location within the protein.
A “nonsense mutation” occurs when a codon change leads to a premature stop codon. This results in a truncated, often non-functional, protein because synthesis halts prematurely. Such changes highlight the importance of accurate codon interpretation for producing proteins with correct structure and function.