[Home]Genetic code

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The genetic code is a translation table for how triplets of adjacent bases, called codons, specify amino acids in protein biosynthesis. In this process, the bases in the DNA from a gene are first copied into a molecule of mRNA during transcription, certain sections of the mRNA are spliced out, and then amino acids are linked to it by molecules of tRNA during translation. Some codons do not specify an amino acid, called STOP codons, which as a result end the translation process.

Nearly all living things use the same genetic code. The standard version is given in the following tables, which show what amino acid each of the 43 = 64 possible codons specify (Table 1), and what codons specify each of the 20 amino acids involved in translation (Table 2). For instance, GAU codes for the amino acid Asp (asparagine?), and Cys (cysteine?) is coded for by the codons UGU and UGC. These are called forward and reverse codon tables, respectively. The bases in the table are adenine?, cytosine, guanine and uracil?, which are used in the mRNA; in the DNA, thymine? takes the place of uracil.

2nd base
U C A G
1st base U UUU Phenylalanine?
UUC Phenylalanine?
UUA Leucine?
UUG Leucine?
UCU Serine?
UCC Serine?
UCA Serine?
UCG Serine?
UAU Tyrosine
UAC Tyrosine
UAA Stop
UAG Stop
UGU Cysteine?
UGC Cysteine?
UGA Stop
UGG Tryptophan?
C CUU Leucine?
CUC Leucine?
CUA Leucine?
CUG Leucine?
CCU Proline?
CCC Proline?
CCA Proline?
CCG Proline?
CAU Histidine?
CAC Histidine?
CAA Glutamine?
CAG Glutamine?
CGU Arginine?
CGC Arginine?
CGA Arginine?
CGG Arginine?
A AUU Isoleucine?
AUC Isoleucine?
AUA Isoleucine?
1AUG Methionine?
ACU Threonine?
ACC Threonine?
ACA Threonine?
ACG Threonine?
AAU Asparagine?
AAC Asparagine?
AAA Lysine?
AAG Lysine?
AGU Serine?
AGC Serine?
AGA Arginine?
AGG Arginine?
G GUU Valine?
GUC Valine?
GUA Valine?
GUG Valine?
GCU Alanine?
GCC Alanine?
GCA Alanine?
GCG Alanine?
GAU [Aspartic acid]?
GAC [Aspartic acid]?
GAA [Glutamic acid]?
GAG [Glutamic acid]?
GGU Glycine
GGC Glycine
GGA Glycine
GGG Glycine
Table 1 : Codon table. This table illustrates the 64 possible codon triplets.
1The AUG codon both codes for methionine and serves as an initiation site; the first AUG in an mRNA's coding region will be the site where translation into protein begins.

Ala GCU, GCC, GCA, GCG Leu UUA, UUG, CUU, CUC, CUA, CUG
Arg CGU, CGC, CGA, CGG, AGA, AGG Lys AAA, AAG
Asn AAU, AAC Met AUG
Asp GAU, GAC Phe UUU, UUC
Cys UGU, UGC Pro CCU, CCC, CCA, CCG
Gln CAA, CAG Ser UCU, UCC, UCA, UCG, AGU, AGC
Glu GAA, GAG Thr ACU, ACC, ACA, ACG
Gly GGU, GGC, GGA, GGG Trp UGG
His CAU, CAC Tyr UAU, UAC
Ile AUU, AUC, AUA Val GUU, GUC, GUA, GUG
START AUG, GUG STOP UAG, UGA, UAA
Table 2 : Reverse codon table. This table shows the 20 amino acids used in proteins, together with the codons that code for them.

In classical genetics, the STOP codons were given names - UAG was amber, UGA was opal, and UAA was ocher. These names were originally the names of the specific genes in which mutation of each of these stop codons was first detected. Translation starts with a chain initiation or START codon, but unlike STOP codons these are not sufficient by themselves to begin the process; nearby initiation sequences are also required to induce transcription into mRNA and binding by ribosomes. The most notable of these is AUG, which also codes for methionine. CUG and UUG, and in prokaryotes GUG and AUU, will also work.

Numerous variations on the standard genetic code are found inside mitochondria, energy-burning organelles that were probably derived from symbiotic bacteria. The Ciliophora or ciliate protozoa also show some variation in the genetic code: UAG and often UAA code for Glutamine, a variant also found in some green algae, or UGA codes for Cysteine. One more variant is found in some species of the yeast? Candida, but interestingly not in all, where CUG codes for Serine.

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Last edited October 27, 2001 6:21 pm by Taw (diff)
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