Stop codon
In genetics, a stop codon or termination codon , also known as nonsense codon , is a codon of ribonucleic acid (RNA) for which there is no associated tRNA (transfer RNA) and which therefore represents the end of a sequence of nucleotides that are attached to ribosomes in the Sequence of amino acids of a polypeptide can be translated.
A stop codon thus determines the end of a reading frame that allows the biosynthesis of a protein and is therefore a necessary condition for a coding nucleic acid segment. The base triplet of a stop codon on an mRNA (messenger RNA) leads to the termination of the translation during protein synthesis in a cell and thus marks - similar to the dot at the end of a word sequence in a sentence - the end of the nucleotide sequence coding for a protein and thus its sentence of amino acids to be linked . The counterpart to the stop codon is the start codon at the beginning of this nucleotide sequence, where translation begins.
Three successive nucleobases , a triplet, form the smallest meaningful unit of the genetic code , called a codon. Each base triplet within an open reading frame codes for one of the proteinogenic amino acids from which a polypeptide chain is built - with the exception of the stop codons. These do not code for any amino acid, since there is no associated tRNA for these codons, but rather they define the end of a reading frame and thus the end of the synthesis of a polypeptide chain for a protein during translation.
In addition to the 61 base triplets of the genetic standard code that code for amino acids, there are three combinations of nucleobases with which the translation can be terminated ; these stop codons are on the mRNA
-
UAGwith UAG = uracil - adenine - guanine -
UGAwith UGA = uracil - guanine - adenine -
UAAwith UAA = uracil - adenine - adenine
The first two codons can also be interpreted in some organisms under special conditions in such a way that they each code for an amino acid. The prerequisite for this is that there is a tRNA loaded with the respective amino acid whose anticodon region binds to the codon on the mRNA. To distinguish it from the usual stop codon, additional circumstances are then necessary, such as certain nucleotide sequences in the vicinity or special RNA structures such as hairpins . In some living beings, such conditions are given that the codon can UAGalso be translated into the amino acid pyrrolysine , or the codon UGAinto the amino acid selenocysteine . These two are therefore - in addition to the canonical twenty - also among the naturally occurring genetically coded proteinogenic amino acids.
The base triplet UAG was after Harris Bernstein as amber called (amber). Subsequently, the triplet UAA was designated as ocher (ocher-colored) and the triplet UGA as opal (opal-colored). These names are an allusion , as the colors have nothing to do physically or chemically with the base triplets.
Mutants
Bacterial UAGmutants whose mRNA contains the codon created by point mutation are also called amber mutants. A compensatory suppressor mutation in the tRNA can enable the protein-synthesizing system to interpret this codon as a sense codon . With the corresponding tRNA gene in the genome of an organism, the codon can UAGthen be translated into a (proteinogenic) amino acid.
If point mutations lead to a stop codon, the result is usually a shortened protein, unless an intron or a section of DNA that does not code for proteins is affected. To convert the start codon AUGinto a stop codon, two or three appropriate point mutations would be required.
