|Index to this page|
One codon, AUG serves two related functions:
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.
|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|
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.
All but two of the amino acids (Met and Trp) can be encoded by from 2 to 6 different codons. However, the genome of most organisms reveals that certain codons are preferred over others. In humans, for example, alanine is encoded by GCC four times as often as by GCG. This probably reflects a greater translation efficiency by the translation apparatus (e.g., ribosomes) for certain codons over their synonyms. [More]
The reason: these mitochondria use UGA to encode tryptophan (Trp) rather than as a chain terminator. When translated by cytosolic machinery, synthesis stops where Trp should have been inserted.
In addition, most
|Link to discussion of mitochondrial genes.|
A few unicellular eukaryotes, notably among the ciliates, have been found that use one or two or even all three of their STOP codons for amino acids. Only those STOP codons occurring close to the poly(A) tail trigger chain termination.
The vast majority of proteins are assembled from the 20 amino acids listed above even though some of these may be chemically altered, e.g. by phosphorylation, at a later time.However, two cases have been found where an amino acid that is not one of the standard 20 is inserted by a tRNA into the growing polypeptide.