Other meanings of Genetic code
Molecular Biology
The genetic code is the set of rules by which information encoded in DNA or RNA sequences is translated into proteins by living cells. It specifies the correspondence between nucleotide triplets, called codons, and amino acids, the building blocks of proteins. This code is nearly universal across all organisms, though minor variations exist in mitochondria and some microbes. Its elucidation in the 1960s is considered a landmark achievement in molecular biology, revealing how genetic information flows from nucleic acids to functional proteins.
The genetic code was cracked in the early 1960s through pioneering experiments by Marshall Nirenberg, Heinrich Matthaei, and Severo Ochoa. Nirenberg and Matthaei used synthetic RNA polymers to show that a chain of uracil (poly-U) directed the synthesis of a protein made only of phenylalanine, establishing UUU as the first codon assigned to an amino acid1. Subsequent work by Har Gobind Khorana and others using repeating copolymers and ribosome-binding assays completed the assignment of all 64 codons. The code is degenerate: most amino acids are specified by more than one codon, which buffers against the effects of point mutations. The standard code is organized so that codons differing in the third base often encode the same amino acid, a property known as wobble.
Translation of the genetic code occurs on ribosomes, where transfer RNA (tRNA) molecules act as adaptors. Each tRNA carries a specific amino acid and bears an anticodon that base-pairs with the mRNA codon. The ribosome catalyzes peptide bond formation between successive amino acids, moving along the mRNA in the 5' to 3' direction. Initiation begins at the start codon AUG, which sets the reading frame; termination occurs when a release factor recognizes a stop codon (UAA, UAG, or UGA), prompting release of the completed polypeptide2. The code is read without overlaps, and the reading frame is maintained by the ribosome's processivity, ensuring accurate protein synthesis.
Although the genetic code is often called universal, several deviations exist. Mitochondrial genomes of vertebrates, invertebrates, and yeast use alternative codons; for example, in human mitochondria, UGA encodes tryptophan instead of a stop signal, and AUA encodes methionine instead of isoleucine3. Some ciliates, such as Tetrahymena, reassign stop codons to amino acids. These variations suggest that the code is not frozen but has evolved, possibly through codon reassignment mechanisms that minimize fitness costs. The near-universality of the standard code is thought to reflect a single origin in the last universal common ancestor, with subsequent divergence in specific lineages.
Beyond the standard 20 amino acids, the code includes selenocysteine and pyrrolysine, the 21st and 22nd amino acids, which are incorporated co-translationally via recoding mechanisms. Selenocysteine is encoded by UGA, normally a stop codon, but requires a downstream stem-loop structure (SECIS element) and a specialized elongation factor4. Pyrrolysine, found in some archaea and bacteria, uses UAG. The code also exhibits codon usage bias: synonymous codons are not used equally, reflecting tRNA abundance and translational efficiency. Additionally, the genetic code is not limited to Earth's biology; astrobiologists study its possible alternatives to understand life's potential elsewhere. The code's origin remains debated, with hypotheses ranging from stereochemical affinities to frozen accident.
The genetic code is a cornerstone of molecular biology, with its near-universality enabling genetic engineering and biotechnology.
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