RNA Codons and DNA Codons

What are codons?

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

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.

Reviewed by Sally Robertson, BSc

References

  1. https://www.umass.edu/microbio/chime/dna/codons.htm
  2. http://www.nobelprize.org/educational/medicine/dna/a/translation/gencode.html
  3. http://waynesword.palomar.edu/codons.htm
  4. http://hyperphysics.phy-astr.gsu.edu/hbase/organic/gencode.html

Further Reading

Last Updated: Mar 3, 2017

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