Codon



A codon is a unit of genetic code. The genetic code is the set of instructions by which information encoded in DNA or RNA sequences is translated into proteins. Most organisms use a nearly universal code that is referred to as the standard genetic code. Even viruses, which are not cellular and do not synthesize proteins themselves, have proteins made using this standard code. Because of this, the code was thought to be universal. However, there are some notable exceptions.

The genetic code consists of 64 triplets of nucleotides. These triplets are called codons. With three exceptions, each codon encodes one of the 20 amino acids used in the synthesis of proteins. This system is very redudant. Most of the amino acids are being encoded by more than one codon. The genetic code is almost universal. The same codons are assigned to the same amino acids and to the same start and stop signals in the vast majority of genes in animals, plants, and microorganisms.

Gene Expression

 * Main Article Gene expression

Gene expression is process through which genetic information is used to construct proteins. Basically specific sequences of DNA act as a template to synthesize mRNA in a process termed transcription. This portion of gene expression occurs in the nucleus. This mRNA is exported from the nucleus into the cytoplasm of the cell and acts as a template to synthesize protein in a process called translation which is catalyzed by the ribosome.

Anticodon

 * Main Article: Translation



An anticodon is one that possesses a complementary or antisense sequence to a codon. Similar to the complementarity between the 2 strands of the DNA molecular, an anticodon is possessed by the transfer RNA.

During the translation of the messenger RNA (mRNA) into a sequence of amino acids, transfer RNAs (tRNA) bring amino acids to the ribosome. The tRNA is itself a codon that must possess a complementary sequence to that of the mRNA. If the sequences or the mRNA and tRNA complement each other, the amino acid carried by the tRNA becomes part of the growing polypeptide.

Start Codons
Three nucleotide bases code for one amino acid in the genetic code. ATG and AUG denote sequences of DNA and RNA that are the start codon or initiation codon encoding the amino acid methionine (Met) in eukaryotes and a modified methionine (fMet) in prokaryotes.

Because AUG is the start codon, methionine is therefore the first amino acid of many proteins. The start codon is almost always preceded by an untranslated region 5' UTR.

Very rarely in higher organisms (eukaryotes) are non AUG start codons used.

Stop Codons
The three stop codons have been given names: UAG is amber, UGA is opal, and UAA is ochre. Amber was named after its discoverer Harris Bernstein, whose last name means amber in German. The other two stop codons were named ochre and opal in order to keep the color theme. Stop codons are also called termination codons, and they signal release of the nascent polypeptide from the ribosome due to binding of release factors in the absence of cognate tRNAs with anticodons complementary to these stop signals.

Reverse codon table
This table shows the 20 standard amino acids used in proteins, and the codons that code for each amino acid.

Related References

 * Kosakovsky Pond S, Delport W,  Muse SV,  Scheffler K, 2010 Correcting the Bias of Empirical Frequency Parameter Estimators in Codon Models. PLoS ONE 5(7): e11230. doi:10.1371/journal.pone.0011230
 * Genetic Code Wikipedia
 * Start Codon Wikipedia
 * Codons Biology Pages