A sense codon is a fundamental unit of genetic information, consisting of a sequence of three nucleotide bases in a messenger RNA (mRNA) molecule. This triplet specifies which of the 20 common amino acids will be added to a protein during its synthesis. This process is part of the central dogma of molecular biology, describing the flow of genetic information from DNA to RNA to protein. The information in DNA is transcribed into an mRNA copy, where sense codons are the “words” read to build proteins.
The Role of Sense Codons in Translation
The function of a sense codon is realized during translation, the cellular process of protein synthesis. This process takes place within the ribosome, a large molecular machine. The ribosome binds to the mRNA molecule and moves along its sequence, reading each three-nucleotide codon. For each sense codon read, a specific transfer RNA (tRNA) molecule is recruited.
Each tRNA molecule acts as an adapter. On one end, it has a three-base sequence called an anticodon, which is complementary to a specific mRNA codon and binds to it. On its other end, the tRNA carries the single amino acid that corresponds to the codon it recognizes.
As the ribosome proceeds along the mRNA, a tRNA with an anticodon matching the current sense codon enters the ribosome. The ribosome then catalyzes the formation of a peptide bond, linking the amino acid from the newly arrived tRNA to the growing polypeptide chain. The ribosome then translocates to the next codon, the now-uncharged tRNA is ejected, and the cycle repeats.
The Genetic Code and Its Redundancy
The genetic code is the set of rules that relates codons to amino acids. With four possible nucleotide bases in mRNA (adenine, uracil, guanine, and cytosine), there are 64 possible three-letter codon combinations (4³). Of these, 61 are sense codons, each specifying an amino acid. The remaining three codons—UAA, UAG, and UGA—are stop codons that signal the termination of translation.
The genetic code has a feature of redundancy, often called degeneracy. Most amino acids are encoded by more than one sense codon. For example, Leucine is specified by six different codons: UUA, UUG, CUU, CUC, CUA, and CUG. This redundancy provides a buffer against some genetic mutations, as a change in the third nucleotide of a codon might not alter the specified amino acid.
One sense codon, AUG, holds a dual responsibility. It is the most common start codon, marking the initiation point for protein synthesis, and it is also a sense codon that specifies the amino acid methionine. Consequently, in eukaryotes, nearly every protein begins with methionine, although it is often removed later.
Consequences of Sense Codon Mutations
Changes to the nucleotide sequence of a sense codon, known as point mutations, can have a wide range of effects on the final protein. These mutations alter the genetic instructions, potentially changing the protein’s structure and function. The outcome depends on how the mutation affects the amino acid sequence.
Due to the redundancy of the genetic code, some mutations are silent. A silent mutation is a change in a single nucleotide that results in a codon that still specifies the same amino acid. For example, if the mRNA codon GCC becomes GCA, the ribosome will still incorporate the amino acid Alanine at that position, leaving the protein’s final amino acid sequence unaffected.
A missense mutation occurs when a nucleotide change alters the codon to one that specifies a different amino acid. If the new amino acid has similar chemical properties to the original, the protein’s function might be only slightly altered. If the properties are very different, the effect can be severe, as seen in sickle-cell anemia. This disease is caused by a single missense mutation changing the codon from GAG to GUG, which substitutes valine for glutamic acid.
For comparison, a nonsense mutation is one where a sense codon is changed into a stop codon (e.g., UAC to UAG). This type of mutation prematurely terminates translation, leading to a shortened and typically nonfunctional protein. This contrasts with sense codon mutations, which alter the protein’s composition rather than truncating it.