codon

A sequence of three base pairs in DNA. There are 64 possible codons (4*4*4). Three of those codons are called "stop codons" because they indicate, in most organisms, the end of a gene.

The three stop codons I know of are "TAA", "TAC", and "TGA".

This page will be updated as I learn more about the stuff I work with.

Groups of three nucleotides found in the coding region of an mRNA molecule. Each codon is translated into a specific amino acid, although due to the redundancy of the genetic code more than one codon can encode the same amino acid.

Every gene in the genome is transcribed into RNA by enzymes called RNA polymerases. RNA is a molecule consisting of a long strand of the nucleotides adenine, guanine, cytosine and uracil (denoted A, G, C and U, respectively). There are three basic types of RNA: transfer RNA, ribosomal RNA and messenger RNA. The first two types perform their own distinct roles in cells (carrying amino acids to the ribosome and catalysing the formation of proteins, respectively), while messenger RNA, or mRNA, serves as the template for the generation of proteins.

Since mRNA is made from nucleic acids and protein is composed of completely different building blocks known as amino acids, there must be a set of rules for translating one into the other. This set of rules is known as the genetic code. Each mRNA molecule contains a 5' untranslated region, a coding region, and a 3' untranslated region. The untranslated regions, or UTRs, serve regulatory functions, while the coding region contains the code that will be translated into protein.

During the process of translation, the codons in the coding region of an mRNA molecule pass in rapid succession through the active site of a ribosome. As each codon enters the active site, a tRNA molecule bearing the appropriate amino acid enters the ribosome and binds to the codon. The ribosome catalyses the formation of a peptide bond between the amino acid and its neighbours in the emerging protein. In this way, the information in the coding region of mRNA is converted into functional proteins, the "doing" molecules of cells.

A codon is a sequence of three bases in DNA or RNA. A codon will code for one of the 20 commonally occuring amino acids, or for start or stop (TAA, TAG, TGA) codons which tell the transcription enzymes when to start and stop DNA transcription and translation. The start codon in humans is ATG, which codes for methionine, which is often removed after protein synthesis. The DNA and RNA codon system is almost completely universal, in that almost every organism uses the same codon to code for the same amino acid.

Here is a table of the 64 possible DNA codons, and the amino acids they code for.

First T      C     A     G   Last 
T     Phe   Ser   Tyr   Cys   T 
      Phe   Ser   Tyr   Cys   C 
      Leu   Ser   Stop  Stop  A 
      Leu   Ser   Stop  Trp   G 
C     Leu   Pro   His   Arg   T 
      Leu   Pro   His   Arg   C   
      Leu   Pro   Gln   Arg   A 
      Leu   Pro   Gln   Arg   G 
A     Ile   Thr   Asn   Ser   T 
      Ile   Thr   Asn   Ser   C 
      Ile   Thr   Lys   Arg   A 
      Met   Thr   Lys   Arg   G 
G     Val   Ala   Asp   Gly   T 
      Val   Ala   Asp   Gly   C 
      Val   Ala   Glu   Gly   A 
      Val   Ala   Glu   Gly   G

A number of exceptions to this do exist: several organisms, most notably archeobacteria, use more than 20 amino acids, (they have 2 extra), and several organisms use different start and stop codons.