Genetic code “Codon” redirects here. For the plant genus, see Codon (genus) . Thegenetic code is the set of rules by which informa- A se ri esof codonsin part of a messenger RNA (mRNA) molecule. Each codon consists of three nucleotides, usually corresponding to a single amino acid. The nucleo tides are abbr evia ted with the letters A, U, G and C. This is mRNA, which uses U ( uracil). DNA us esT (thymine) instead. Thi s mRNA mol ecu le wil l instructaribosometo synthesize a protein according to this code. tion encoded within genetic material ( DNA or mRNA sequences) istranslatedintoproteinsby livingcells. Bio- logical decoding is accomplished by the ribosome, which linksamino acids in an order specified by mRNA, using transfer RNA (tRNA) mol ec ul es to ca rry amino acid s and to read the mRNA three nucleotides at a time. The ge- netic code is highly similar among all organisms and can be expressed in a simple table with 64 entries. The code defines how sequences of nucleotide triplets, called codons, specify which amino acid will be added next duringprotein synthesis. With some excep tions, [1] a three-nucleotide codon in a nucleic acid sequence spec- ifie s a sin gle amino acid . Beca use the vast ma jori ty of genes are encoded with exactly the same code (see the RNA codon table), this particular code is often referred to as the canonical or standard genetic code, or sim- plythe genetic code, though in fact some variant codes have evol ved. For example , protein synthesis in human mitochondria relies on a genetic code that differs from the standard genetic code. While the genetic code determines the protein sequence for a given coding region, other genomic regions can in- fluencewhen and where these proteins are produced. 1 Di sc over y G A A A A A G G G G C C C C U U U U UCAG U CA G U C A G U C A G U C A G U C A G U C A G U C A G U C A G U C A G U C A G U C A G U C A G U C A G U C A GUCAG P S U nG nG oG oG oG G P P P P P nM nM M M nM nM nM Phenylalanine Phe Leucine Leu Leucine Leu Proline Pro Histidine His Glutamine Gln Isoleucine Ile Methionine Met Threonine Thr Asparagine Asn Lysine Lys Arginine Arg Arginine Arg Valine Val Alanine Ala Glutamic acid Glu Aspartic acid Asp Glycine Gly Serine Ser Serine Ser Tyrosi ne T yr Cysteine Cys Trypt ophan T rp Stops Stop E G FL S S Y C W L P H R R Q I M T N KV A D 89.09 75.07 174.20 174.20 146.19 165.19 133.11 117.15 147.13 146.15 155.16 115.13 105.09 105.09 131.18 132.12 M W = 1 4 9 . 2 1 D a 131.18 119.12 204.23 131.18 181.19 121.16 HN NH2 NH H2N OH O H2N C H3 OH O H2N O H2N OH O O HO H2N OH O HS H2N OH O H2N O NH2 OH O O OH H2N OH O H2N OH O NH H2N OH O N C H3 CH3 H2N OH O C H3 C H3 H2N OH O C H3 C H3 H2N OH O H2N H2N OH O C H3 S H2N OH O H2N OH O NH OH O H2N HO OH O H2N HO OH O H2N HO CH3 OH O NH H2N OH O HO H2N OH O H2N C H3 CH3 OH O Basic Acidic Polar Nonpolar (hydrophobic) S- M - P - U - nM - oG - nG - Sumo Methyl Phospho Ubiquitin N-Methyl O-glycosyl N-glycosyl Modification a m in o a cid2nd1stposition 3rdU C The genetic code Serious efforts to understand how proteins are encoded beg an afterthe str uc tur e of DNA wa s disco ve red in 1953. George Gamowpostulated that sets of three bases must be employed to encode the 20 standard amino acids used by living cells to build proteins. With four different nu- 1
