A codon table can be used to translate a
genetic code
The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accomplished by the ribosome, which links ...
into a sequence of
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s.
The standard genetic code is traditionally represented as an
RNA codon table, because when
proteins are made in a
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
by
ribosomes, it is
messenger RNA (mRNA) that directs
protein synthesis
Protein biosynthesis (or protein synthesis) is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critical ...
.
The mRNA sequence is determined by the sequence of genomic
DNA. In this context, the standard genetic code is referred to as translation table 1.
It can also be represented in a DNA codon table. The DNA codons in such tables occur on the
sense DNA strand and are arranged in a
5′-to-3′ direction. Different tables with alternate codons are used depending on the source of the genetic code, such as from a
cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
,
mitochondrion,
plastid, or
hydrogenosome
A hydrogenosome is a membrane-enclosed organelle found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular hydrogen and ATP i ...
.
There are 64 different codons in the genetic code and the below tables; most specify an amino acid. Three sequences, UAG, UGA, and UAA, known as
stop codon
In molecular biology (specifically protein biosynthesis), a stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that signals the termination of the translation process of the current protein. Most codons in m ...
s, do not code for an amino acid but instead signal the release of the nascent
polypeptide from the ribosome.
In the standard code, the sequence AUG—read as
methionine—can serve as a
start codon
The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and Archaea and a N-formylmethionine (fMet) in bacteria, mitochondria and plastids. The m ...
and, along with sequences such as an
initiation factor, initiates translation.
In rare instances, start codons in the standard code may also include GUG or UUG; these codons normally represent
valine and
leucine
Leucine (symbol Leu or L) is an essential amino acid that is used in the biosynthesis of proteins. Leucine is an α-amino acid, meaning it contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α- c ...
, respectively, but as start codons they are translated as
methionine or
formylmethionine.
The first table—the standard table—can be used to translate
nucleotide triplets into the corresponding amino acid or appropriate signal if it is a start or stop codon. The second table, appropriately called the inverse, does the opposite: it can be used to deduce a possible triplet code if the amino acid is known. As multiple codons can code for the same amino acid, the
International Union of Pure and Applied Chemistry's (IUPAC)
nucleic acid notation is given in some instances.
Translation table 1
Standard RNA codon table
Inverse RNA codon table
Standard DNA codon table
Inverse DNA codon table
Alternative codons in other translation tables
The genetic code was once believed to be universal:
a codon would code for the same amino acid regardless of the organism or source. However, it is now agreed that the genetic code evolves,
resulting in discrepancies in how a codon is translated depending on the genetic source.
For example, in 1981, it was discovered that the use of codons AUA, UGA, AGA and AGG by the coding system in mammalian mitochondria differed from the universal code.
Stop codons can also be affected: in
ciliated protozoa, the universal stop codons UAA and UAG code for glutamine.
The following table displays these alternative codons.
See also
*
Bioinformatics
Bioinformatics () is an interdisciplinary field that develops methods and software tools for understanding biological data, in particular when the data sets are large and complex. As an interdisciplinary field of science, bioinformatics combine ...
*
List of genetic codes
While there is much commonality, different parts of the tree of life use slightly different genetic codes. When translating from genome to protein, the use of the correct genetic code is essential. The mitochondrial codes are the relatively well-k ...
Notes
References
Further reading
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External links
DNA codon chart organized in a wheel
{{featured list
Gene expression
Molecular genetics
Protein biosynthesis
Bioinformatics