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Codon

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RNA codons
Codon translator.

A codon is a unit of genetic code. The genetic code is the set of instructions by which information encoded in DNA or RNA sequences is translated into proteins. Most organisms use a nearly universal code that is referred to as the standard genetic code. Even viruses, which are not cellular and do not synthesize proteins themselves, have proteins made using this standard code. Because of this, the code was thought to be universal. However, there are some notable exceptions.[1]

The genetic code consists of 64 triplets of nucleotides. These triplets are called codons. With three exceptions, each codon encodes one of the 20 amino acids used in the synthesis of proteins. This system is very redudant. Most of the amino acids are being encoded by more than one codon. The genetic code is almost universal. The same codons are assigned to the same amino acids and to the same start and stop signals in the vast majority of genes in animals, plants, and microorganisms.[2]

Contents

Gene Expression

Main Article Gene expression

Gene expression is process through which genetic information is used to construct proteins. Basically specific sequences of DNA act as a template to synthesize mRNA in a process termed transcription. This portion of gene expression occurs in the nucleus. This mRNA is exported from the nucleus into the cytoplasm of the cell and acts as a template to synthesize protein in a process called translation which is catalyzed by the ribosome.

Anticodon

Main Article: Translation
tRNA anticodon complements the mRNA codon.

An anticodon is one that possesses a complementary or antisense sequence to a codon. Similar to the complementarity between the 2 strands of the DNA molecular, an anticodon is possessed by the transfer RNA.

During the translation of the messenger RNA (mRNA) into a sequence of amino acids, transfer RNAs (tRNA) bring amino acids to the ribosome. The tRNA is itself a codon that must possess a complementary sequence to that of the mRNA. If the sequences or the mRNA and tRNA complement each other, the amino acid carried by the tRNA becomes part of the growing polypeptide.

Start Codons

Three nucleotide bases code for one amino acid in the genetic code. ATG and AUG denote sequences of DNA and RNA that are the start codon or initiation codon encoding the amino acid methionine (Met) in eukaryotes and a modified methionine (fMet) in prokaryotes.

Because AUG is the start codon, methionine is therefore the first amino acid of many proteins. The start codon is almost always preceded by an untranslated region 5' UTR.

Very rarely in higher organisms (eukaryotes) are non AUG start codons used.[3]

Stop Codons

The three stop codons have been given names: UAG is amber, UGA is opal, and UAA is ochre. Amber was named after its discoverer Harris Bernstein, whose last name means amber in German. The other two stop codons were named ochre and opal in order to keep the color theme. Stop codons are also called termination codons, and they signal release of the nascent polypeptide from the ribosome due to binding of release factors in the absence of cognate tRNAs with anticodons complementary to these stop signals.[4]

RNA codon table

This table shows the 64 codons and the amino acid each codon codes for. The direction is 5' to 3'.
2nd base
U C A G
1st
base
U

UUU (Phe/F)Phenylalanine
UUC (Phe/F)Phenylalanine
UUA (Leu/L)Leucine
UUG (Leu/L)Leucine

UCU (Ser/S)Serine
UCC (Ser/S)Serine
UCA (Ser/S)Serine
UCG (Ser/S)Serine

UAU (Tyr/Y)Tyrosine
UAC (Tyr/Y)Tyrosine
UAA Ochre (Stop)
UAG Amber (Stop)

UGU (Cys/C)Cysteine
UGC (Cys/C)Cysteine
UGA Opal (Stop)
UGG (Trp/W)Tryptophan

C

CUU (Leu/L)Leucine
CUC (Leu/L)Leucine
CUA (Leu/L)Leucine
CUG (Leu/L)Leucine

CCU (Pro/P)Proline
CCC (Pro/P)Proline
CCA (Pro/P)Proline
CCG (Pro/P)Proline

CAU (His/H)Histidine
CAC (His/H)Histidine
CAA (Gln/Q)Glutamine
CAG (Gln/Q)Glutamine

CGU (Arg/R)Arginine
CGC (Arg/R)Arginine
CGA (Arg/R)Arginine
CGG (Arg/R)Arginine

A

AUU (Ile/I)Isoleucine
AUC (Ile/I)Isoleucine
AUA (Ile/I)Isoleucine
AUG (Met/M)Methionine(Start)

ACU (Thr/T)Threonine
ACC (Thr/T)Threonine
ACA (Thr/T)Threonine
ACG (Thr/T)Threonine

AAU (Asn/N)Asparagine
AAC (Asn/N)Asparagine
AAA (Lys/K)Lysine
AAG (Lys/K)Lysine

AGU (Ser/S)Serine
AGC (Ser/S)Serine
AGA (Arg/R)Arginine
AGG (Arg/R)Arginine

G

GUU (Val/V)Valine
GUC (Val/V)Valine
GUA (Val/V)Valine
GUG (Val/V)Valine

GCU (Ala/A)Alanine
GCC (Ala/A)Alanine
GCA (Ala/A)Alanine
GCG (Ala/A)Alanine

GAU (Asp/D)Aspartic acid
GAC (Asp/D)Aspartic acid
GAA (Glu/E)Glutamic acid
GAG (Glu/E)Glutamic acid

GGU (Gly/G)Glycine
GGC (Gly/G)Glycine
GGA (Gly/G)Glycine
GGG (Gly/G)Glycine

Reverse codon table

This table shows the 20 standard amino acids used in proteins, and the codons that code for each amino acid.[5]

Ala A GCU, GCC, GCA, GCG Leu L UUA, UUG, CUU, CUC, CUA, CUG
Arg R CGU, CGC, CGA, CGG, AGA, AGG Lys K AAA, AAG
Asn N AAU, AAC Met M AUG
Asp D GAU, GAC Phe F UUU, UUC
Cys C UGU, UGC Pro P CCU, CCC, CCA, CCG
Gln Q CAA, CAG Ser S UCU, UCC, UCA, UCG, AGU,AGC
Glu E GAA, GAG Thr T ACU, ACC, ACA, ACG
Gly G GGU, GGC, GGA, GGG Trp W UGG
His H CAU, CAC Tyr Y UAU, UAC
Ile I AUU, AUC, AUA Val V GUU, GUC, GUA, GUG
Start AUG Stop UAG, UGA, UAA

Related References

  • Kosakovsky Pond S, Delport W, Muse SV, Scheffler K, 2010 Correcting the Bias of Empirical Frequency Parameter Estimators in Codon Models. PLoS ONE 5(7): e11230. doi:10.1371/journal.pone.0011230 [6]
  • Genetic Code Wikipedia
  • Start Codon Wikipedia
  • Codons Biology Pages
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