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.
References
- https://www.umass.edu/microbio/chime/dna/codons.htm
- http://www.nobelprize.org/educational/medicine/dna/a/translation/gencode.html
- http://waynesword.palomar.edu/codons.htm
- http://hyperphysics.phy-astr.gsu.edu/hbase/organic/gencode.html
Further Reading
- All DNA Content
- What is DNA?
- DNA Properties
- DNA Chemical Modifications
- DNA Biological Functions
Last Updated: Feb 26, 2019
Written by
Susha Cheriyedath
Susha has a Bachelor of Science (B.Sc.) degree in Chemistry and Master of Science (M.Sc) degree in Biochemistry from the University of Calicut, India. She always had a keen interest in medical and health science. As part of her masters degree, she specialized in Biochemistry, with an emphasis on Microbiology, Physiology, Biotechnology, and Nutrition. In her spare time, she loves to cook up a storm in the kitchen with her super-messy baking experiments.
Source: Read Full Article