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This table shows the 64 codons and the amino acid each codon
codes for. The direction is 5' to 3'
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| UUU
(Phe/F) UUC (Phe/F) UUA (Leu/L) UUG (Leu/L) |
UCU
(Ser/S)
UCC (Ser/S)
UCA (Ser/S)
UCG (Ser/S) |
UAU
(Tyr/Y)
UAC (Tyr/Y)
UAA (Ochr)
UAG (Amber) |
UGU
(Cys/C)
UGC (Cys/C)
UGA (Opal)
UGG (Trp/W) |
| CUU
(Leu/L) CUC (Leu/L) CUA (Leu/L) CUG (Leu/L) |
CCU
(Pro/P)
CCC (Pro/P)
CCA (Pro/P)
CCG (Pro/P) |
CAU
(His/H)
CAC (His/H)
CAA (Gln/Q)
CAG (Gln/Q) |
CGU
(Arg/R)
CGC (Arg/R)
CGA (Arg/R)
CGG (Arg/R)
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| AUU
(Ile/I) AUC (Ile/I) AUA (Ile/I) AUG
(Met/M) |
ACU
(Thr/T)
ACC (Thr/T)
ACA (Thr/T)
ACG (Thr/T) |
AAU
(Asn/N)
AAC (Asn/N)
AAA (Lys/K)
AAG (Lys/K) |
AGU
(Ser/S)
AGC (Ser/S)
AGA (Arg/R)
AGG (Arg/R)
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| GUU
(Val/V) GUC (Val/V) GUA (Val/V) GUG (Val/V) |
GCU
(Ala/A)
GCC (Ala/A)
GCA (Ala/A)
GCG (Ala/A) |
GAU
(Asp/D)
GAC (Asp/D)
GAA (Glu/E)
GAG (Glu/E) |
GGU
(Gly/G)
GGC (Gly/G)
GGA (Gly/G)
GGG (Gly/G)
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Termination
Codon |
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Initiation
Codon |
An explanation of the Genetic Code:
DNA is a two-stranded molecule. One strand of DNA holds
the information that codes for various genes; this strand
is often called the template strand or antisense strand (containing
anticodons). The other, and complementary, strand is called
the coding strand or sense strand (containing codons). Since
mRNA is made from the template strand, it has the same information
as the coding strand.
The table above refers to triplet nucleotide codons along
the sequence of the coding or sense strand of DNA as it runs
5' -> 3'; the code for the mRNA would be identical but for
the fact that RNA contains U (uridine) rather than T.
An example of two complementary strands of DNA would be:
(5' -> 3') ATGGAATTCTCGCTC (Coding, sense strand)
(3' <- 5') TACCTTAAGAGCGAG (Template, antisense strand)
(5' -> 3') AUGGAAUUCUCGCUC (mRNA made from Template strand)
Since amino acid residues of proteins are specified as triplet
codons, the protein sequence made from the above example would
be Met-Glu-Phe-Ser-Leu... (MEFSL...).
Practically, codons are "decoded" by transfer RNAs (tRNA)
which interact with a ribosome-bound messenger RNA (mRNA)
containing the coding sequence. There are 64 different tRNAs,
each of which has an anticodon loop (used to recognize codons
in the mRNA). 61 of these have a bound amino acyl residue;
the appropriate "charged" tRNA binds to the respective next
codon in the mRNA and the ribosome catalyzes the transfer
of the amino acid from the tRNA to the growing (nascent) protein/polypeptide
chain.
The remaining 3 codons are used for "punctuation"; that is,
they signal the termination (the end) of the growing polypeptide
chain. Lastly, the Genetic Code in the table above has also
been called "The Universal Genetic Code". It is known as "universal",
because it is used by all known organisms as a code for DNA,
mRNA, and tRNA. The universality of the genetic code encompases
animals (including humans), plants, fungi, archaea, bacteria,
and viruses. However, all rules have their exceptions, and
such is the case with the Genetic Code; small variations in
the code exist in mitochondria and certain microbes. Nonetheless,
it should be emphasized that these variances represent only
a small fraction of known cases, and that the Genetic Code
applies quite broadly, certainly to all known nuclear genes.
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