The codon chart is a universal reference tool that serves as the dictionary for the genetic code. It displays the triplet sequences found on messenger RNA (mRNA) and the corresponding amino acid each sequence specifies. This genetic dictionary guides translation, the process where the nucleotide sequence of a gene is converted into the amino acid sequence of a protein. Understanding this chart is foundational to comprehending how genetic information flows from DNA to its final product in a cell.
Decoding the Genetic Message on Messenger RNA
Messenger RNA is the single-stranded nucleic acid molecule that carries the genetic instructions copied from DNA in the nucleus out to the cytoplasm, where proteins are made. The sequence of bases on the mRNA molecule is read in specific units of three nucleotides, and each of these three-base units is called a codon. With four different RNA bases—Adenine (A), Uracil (U), Cytosine (C), and Guanine (G)—there are 64 possible combinations of these triplet codons.
The sequence of codons on the mRNA molecule is read sequentially in a specific order known as the reading frame. This frame is established by a particular start codon, typically AUG, which signals the beginning of the protein sequence. If the ribosome shifts this frame by even a single nucleotide, the entire subsequent sequence of amino acids will be incorrect.
The Function of Transfer RNA and the Anticodon
While the codon chart is based on the mRNA sequence, another molecule, transfer RNA (tRNA), is the physical adapter that effectively reads and translates the code. Each tRNA molecule functions like a molecular truck, carrying a specific amino acid on one end. On the opposite end of the tRNA molecule is a specific three-nucleotide sequence called the anticodon.
The anticodon sequence is complementary to a specific codon on the mRNA strand. For example, if an mRNA codon is AUG, the corresponding tRNA anticodon that recognizes it will be UAC. The tRNA brings the correct amino acid to the ribosome and matches it with its designated mRNA codon.
Reading the Chart: The Translation Process
Translation takes place on the ribosome, which acts as a platform where the mRNA and tRNA molecules interact. The ribosome moves along the mRNA strand, reading it one codon at a time from the 5′ end to the 3′ end. As the ribosome encounters an mRNA codon, the tRNA carrying the appropriate amino acid enters and temporarily binds by matching its anticodon to the mRNA codon.
Once the correct tRNA is in place, the ribosome catalyzes the formation of a peptide bond, linking the newly delivered amino acid to the end of the growing polypeptide chain. A slight flexibility in base pairing at the third position of the codon-anticodon interaction is known as the “wobble” hypothesis. This wobble allows a single type of tRNA to recognize and bind to multiple, synonymous codons that code for the same amino acid, which allows organisms to efficiently synthesize proteins.
Universality and Variations in the Code
The genetic code displayed by the codon chart is considered nearly universal, meaning the same codon sequences specify the same amino acids across almost all forms of life. This universality suggests the code originated early in life’s history and has been strongly conserved. The code is also described as degenerate, meaning that multiple codons often code for the same single amino acid.
The degeneracy of the code acts as a buffer against potential mutations, as a change in a single nucleotide may still result in the same amino acid being added to the protein. Among the 64 possible codons, 61 code for the 20 common amino acids. The remaining three—UAA, UAG, and UGA—are stop codons that signal the termination of protein synthesis. The single codon AUG serves a dual purpose, coding for the amino acid methionine and also acting as the start signal for translation.