Stopcodon

As a stop codon, also nonsense codon is a triplet of bases (codon ) is called deoxyribonucleic acid ( DNA) or ribonucleic acid (RNA) in genetics, for which no cognate tRNA ( transfer RNA ) is present and therefore the termination of translation or protein biosynthesis ( protein synthesis ) results in a cell. The counterpart to the stop codon is the initiation codon, there begins the translation.

Three consecutive nucleobases, a so-called base triplet (also called codon ) are the smallest unit of the genetic code. Each base triplet in an ORF on the mRNA ( messenger RNA ) encoding one of the amino acids from which proteins are constructed - with the exception of the stop codon. They code for any amino acid, as no cognate tRNA to these codons is present. Thus stop codons cause the termination of translation and thus the synthesis of the protein.

In addition to the 61 amino acid encoding triplet base of the universal genetic code, there are three combinations of nucleobases which terminate protein synthesis, the stop codons:

• UAA = uracil - adenine - adenine
• UAG = uracil - adenine - guanine
• UGA = uracil - guanine - adenine

The base triplet UAG was designated by Harris Bernstein as amber (amber ). As a result, the triplet UAA was designated as ocher ( ocher ) and the triplet UGA as opal ( opal ). The names are an allusion, as the colors have nothing to do with the base triplets.

Mutants

Mutant bacteria, the resulting mRNA contains the codon UAG by point mutation are also called Amber mutants. A compensatory suppressor mutation in the tRNA can, however, enable the protein-synthesizing system to interpret the stop codon as a sense codon. Add to that an organism's genome, a corresponding tRNA gene added, the corresponding stop codon can be translated to an amino acid. Point mutations may result in a stop codon and thus to a truncated protein.