The Genetic Code

The relationship between the codons of nucleic acids, and the amino acids for which they code, is embodied in the Genetic Code. It is nearly universal as only slight variations on it are found in mitochondria and chloroplasts. A codon consists of a triplet of bases. There are 64 possible combinations. It took from 1961 to 1967 for Marshall Nirenberg and co-workers to "crack" the code. For this they received the 1968 Nobel Prize in Medicine & Physiology.

UUU Phe UUC Phe UUA Leu UUG Leu	UCU Ser UCC Ser UCA Ser UCG Ser	UAU Tyr UAC Tyr UAA STOP UAG STOP	UGU Cys UGC Cys UGA STOP UGG Trp
CUU Leu CUC Leu CUA Leu CUG Leu	CCU Pro CCC Pro CCA Pro CCG Pro	CAU His CAC His CAA Gln CAG Gln	CGU Arg CGC Arg CGA Arg CGG Arg
AUU Ile AUC Ile AUA Ile AUG Met	ACU Thr ACC Thr ACA Thr ACG Thr	AAU Asn AAC Asn AAA Lys AAG Lys	AGU Ser AGC Ser AGA Arg ACG Arg
GUU Val GUC Val GUA Val GUG Val	GCU Ala GCC Ala GCA Ala GCG Ala	GAU Asp GAC Asp GAA Glu GAG Glu	GGU Gly GGC Gly GGA Gly GGG Gly

In most cases sufficient coding is performed by the first two bases. The third (or wobble) base plays a minor but oftentimes significant role.

Methionine (met) is always the first amino acid in a newly synthesized protein as AUG is the codon that signals the beginning of a chain. UAA, UAG, and UGA are the STOP codons, which cause termination of translation by the ribosome.

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