The Genetic Code

The Genetic Code

Index to this page

* The RNA Codons

* The DNA Codons

* Codon Bias

* Exceptions to the Code

The genetic code consists of 64 triplets of nucleotides. These triplets are called codons.With three exceptions, each codon encodes for one of the 20 amino acids used in the synthesis of proteins. That produces some redundancy in the code: most of the amino acids being encoded by more than one codon.

One codon, AUG serves two related functions:

* it signals the start of translation

* it codes for the incorporation of the amino acid methionine (Met) into the growing polypeptide chain

The genetic code can be expressed as either RNA codons or DNA codons. RNA codons occur in messenger RNA (mRNA) and are the codons that are actually "read" during the synthesis of polypeptides (the process called translation). But each mRNA molecule acquires its sequence of nucleotides by transcription from the corresponding gene. Because DNA sequencing has become so rapid and because most genes are now being discovered at the level of DNA before they are discovered as mRNA or as a protein product, it is extremely useful to have a table of codons expressed as DNA. So here are both.

Note that for each table, the left-hand column gives the first nucleotide of the codon, the 4 middle columns give the second nucleotide, and the last column gives the third nucleotide.

The RNA Codons

Second nucleotide

U C A G

U UUU Phenylalanine (Phe) UCU Serine (Ser) UAU Tyrosine (Tyr) UGU Cysteine (Cys) U

UUC Phe UCC Ser UAC Tyr UGC Cys C

UUA Leucine (Leu) UCA Ser UAA STOP UGA STOP A

UUG Leu UCG Ser UAG STOP UGG Tryptophan (Trp) G

C CUU Leucine (Leu) CCU Proline (Pro) CAU Histidine (His) CGU Arginine (Arg) U

CUC Leu CCC Pro CAC His CGC Arg C

CUA Leu CCA Pro CAA Glutamine (Gln) CGA Arg A

CUG Leu CCG Pro CAG Gln CGG Arg G

A AUU Isoleucine (Ile) ACU Threonine (Thr) AAU Asparagine (Asn) AGU Serine (Ser) U

AUC Ile ACC Thr AAC Asn AGC Ser C

AUA Ile ACA Thr AAA Lysine (Lys) AGA Arginine (Arg) A

AUG Methionine (Met) or START ACG Thr AAG Lys AGG Arg G

G GUU Valine Val GCU Alanine (Ala) GAU Aspartic acid (Asp) GGU Glycine (Gly) U

GUC (Val) GCC Ala GAC Asp GGC Gly C

GUA Val GCA Ala GAA Glutamic acid (Glu) GGA Gly A

GUG Val GCG Ala GAG Glu GGG Gly G

________________________________________

The DNA Codons

These are the codons as they are read on the sense (5' to 3') strand of DNA. Except that the nucleotide thymidine (T) is found in place of uridine (U), they read the same as RNA codons. However, mRNA is actually synthesized using the antisense strand of DNA (3' to 5') as the template. [Discussion]

This table could well be called the Rosetta Stone of life.

The Genetic Code (DNA)

TTT Phe TCT Ser TAT Tyr TGT Cys

TTC Phe TCC Ser TAC Tyr TGC Cys

TTA Leu TCA Ser TAA STOP TGA STOP

TTG Leu TCG Ser TAG STOP TGG Trp

CTT Leu CCT Pro CAT His CGT Arg

CTC Leu CCC Pro CAC His CGC Arg

CTA