The genetic code is made up of a total of 64 base triplets or codons. At least one codon encodes the information for each of the 20 amino acids used in the synthesis of proteins during translation. While one codon can code for only one amino acid, more than one codon can code for the same amino acid, which is described as the degeneracy of the code. The majority of amino acids are encoded for by more than one codon.
START and STOP codons
AUG is the most common START codon, which signals the beginning of translation. It codes for the amino acid methionine (Met) and directs the addition of Met to the growing polypeptide chain during protein synthesis. Of the 64 codons, only 61 code for amino acids and the remaining three codons are STOP codons that signal the end or termination of translation. UAA, UAG, and UGA are the three RNA STOP codons and TAG, TAA and TGA are the three DNA stop codons.
RNA codons
Traditionally, the genetic code was represented by RNA codons, as it is messenger RNA (mRNA) that directs translation. Codons in the mRNA are decoded by transfer RNA (tRNA) during protein synthesis.
RNA codons and the amino acids they encode are tabulated below:
UUU Phe
UCU Ser
UAU Tyr
UGU Cys
UUC Phe
UCC Ser
UAC Tyr
UGC Cys
UUA Leu
UCA Ser
UAA STOP
UGA STOP
UUG Leu
UCG Ser
UAG STOP
UGG Trp
CUU Leu
CCU Pro
CAU His
CGU Arg
CUC Leu
CCC Pro
CAC His
CGC Arg
CUA Leu
CCA Pro
CAA Gln
CGA Arg
CUG Leu
CCG Pro
CAG Gln
CGG Arg
AUU Ile
ACU Thr
AAU Asn
AGU Ser
AUC Ile
ACC Thr
AAC Asn
AGC Ser
AUA Ile
ACA Thr
AAA Lys
AGA Arg
AUG Met / START codon
ACG Thr
AAG Lys
AGG Arg
GUU Val
GCU Ala
GAU Asp
GGU Gly
GUC Val
GCC Ala
GAC Asp
GGC Gly
GUA Val
GCA Ala
GAA Glu
GGA Gly
GUG Val
GCG Ala
GAG Glu
GGG Gly
Key: Phe = Phenylalanine, Ser = Serine, Tyr = Tyrosine, Cys = Cysteine, Leu = Leucine, Trp = Tryptophan, Pro = Proline, Gln = Glutamine, Asparagine = ASN, Thr = Threonine, Ile = Isoleucine, Asp = Aspartic acid, Glu = Glutamic acid, Gly = Glycine, Ala = Alanine, Val = Valine, Met = Methionine, Arg = Arginine, His = Histidine, Lys = Lysine.
As a result of advances in genomics and computational technology, genes are mostly now discovered at the DNA level, before conversion to mRNA and proteins and it has become increasingly popular to use DNA codons. The DNA codons are identical to the RNA codons, except for the one base thymine (T), which replaces uracil (U) in the RNA codons.
The DNA codons and the amino acids they represent are tabulated below:
Codon
Amino acid
Codon
Amino acid
TTT
Phenylalanine (Phe)
TCT
Serine (Ser)
TTC
TCC
TTA
Leucine (Leu)
TCA
TTG
TCG
CTT
CCT
Proline (Pro)
CTC
CCC
CTA
CCA
CTG
CCG
ATT
Isoleucine (Ile)
ACT
Threonine (Thr)
ATC
ACC
ATA
ACA
ATG
Methionine (Met) / START codon
ACG
GTT
Valine (Val)
GCT
Alanine (Ala)
GTC
GCC
GTA
GCA
GTG
GCG
TAT
Tyrosine (Tyr)
TGT
Cysteine (Cys)
TAC
TGC
TAA
STOP codon (Ochre)
TGA
STOP (Opal)
TAG
STOP codon (Amber)
TGG
Tryptophan (Trp)
CAT
Histidine (His)
CGT
Arginine (Arg)
CAC
CGC
CAA
Glutamine (Gln)
CGA
CAG
CGG
AAT
Asparagine (Asn)
AGT
Serine (Ser)
AAC
AGC
AAA
Lysine (Lys)
AGA
Arginine (Arg)
AAG
AGG
GAT
Aspartic acid (Asp)
GGT
Glycine (Gly)
GAC
GGC
GAA
Glutamic acid (Glu)
GGA
GAG
GGG
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As can be seen from the above tables, most of the amino acids are encoded by multiple codons. Asn, Asp, Cys, Gln, Glu, His, Lys, Phe, and Tyr have two codons; Ile has three codons; Ala, Gly, Pro, Thr, and Val have four codons; and Arg, Leu, and Ser have six codons. Only two amino acids - Met and Trp – are encoded by a single codon each.
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//www.britannica.com/science/codonFeedbackAlternate titles: trinucleotide
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Table of ContentsKey People:Francis Crick Marshall Warren Nirenberg...(Show more)Related Topics:genetic code...(Show more)
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codon, in genetics, any of 64 different sequences of three adjacent nucleotides in DNA that either encodes information for the production of a specific amino acid or serves as a stop signal to terminate translation (protein synthesis). Codons are made up of any triplet combination of the four nitrogenous bases adenine (A), guanine (G), cytosine (C), or uracil (U). Of the 64 possible codon sequences, 61 specify the 20 amino acids that make up proteins and three are stop signals.
An example of a codon is the sequence AUG, which specifies the amino acid methionine. The AUG codon, in addition to coding for methionine, is found at the beginning of every messenger RNA (mRNA) and indicates the start of a protein. Methionine and tryptophan are the only two amino acids that are coded for by just a single codon (AUG and UGG, respectively). The other 18 amino acids are coded for by two to six codons. Because most of the 20 amino acids are coded for by more than one codon, the genetic code is called degenerate. The same codons specify the same amino acids in almost all species.
Examples of termination codons are UAG, UAA, or UGA. Translation stops when one of these codons is encountered by the ribosome (ribosomes are small particles in cells that serve as the sites of protein synthesis). Special release factors associate with the ribosome in response to these codons, and the newly synthesized protein, transfer RNAs (tRNAs), and mRNA dissociate.