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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