Nonsense codon

Codon A set of three nucleotides in a protein coding sequence that specifies individual amino acids or a termination signal (codon, terminator). Most codons are universal, but some organisms do not produce the transfer RNAs (RNA, transfer) complementary to all codons. These codons are referred to as unassigned codons (codons, nonsense). [NIH]... 

Stop codons. (termination codons nonsense codons). UAA, UAG, and UGA. In protein synthesis these codons signal the termination of a polypeptide chain.. 

Amber codon, nonsense codon the sequence UAG in mRNA. It does not code for any of the 20 proteogenic amino acids, and it results in the termination of protein synthesis (premature termination when UAG is produced by mutation of a sense codon). Potential precursors for the production of UAG by mutation are UCG (serine), UAU and UAC (tyrosine) and CAG (glutamine). 

Most of the aforementioned efforts are based on the nonsense suppression methodology as the method for peptide altering [100]. In this approach, a nonsense codon is introduced into the enzyme-coding mRNA in the site that has to be altered. Simultaneously, the tRNA-noncoded amino acid hybrids are prepared with nonsense anticodons. Finally, the translation of modified mRNA is performed in vivo [101] or in vitro [102]. 

A nonsense codon may appear that would then result in the premature termination of amino acid incorporation into a peptide chain and the production of only a fragment of the intended protein molecule. The probabihty is high that a premamrely terminated protein molecule or peptide fragment will not function in its assigned role. 

C9. Cheng, J., and Maquat, L. E., Nonsense codons can reduce the abundance of nuclear mRNA without affecting the abundance of pre-mRNA or the half-life of cytoplasmic mRNA. Mol. Cell. Biol. 13,1892-1902 (1993). 

Nonsense mutations change normal codons into nonsense codons. In viruses, nonsense mutations are recognized because hosts are available that contain suppressors able to read nonsense codons. The virus mutant will be able to grow in the host containing the suppressor, but not in the normal host. 

We have not directly determined the relative frequencies of frameshift mutations and other mutational events in comparison to the base substitution mutations. However, based on the high frequency of nonsense mutations (11%) among all lacl mutants induced by BPDE and because nonsense mutations are monitorable at less than one-fifth of the lacl codons and, even then, only via certain base pair substitutions, we believe that base substitutions account for a major fraction of mutations induced by BPDE. 

Sites at which nonsense mutations are detected are identified by their amber (A) or ochre (0) alleles (Coulondre and Miller, 1977). The 8 tyrosine codons in lacl each have two nonsense alleles, one amber and one ochre. The amber alleles at these sites are marked by the symbols and . 

VAN HOOF, A., GREEN, P.J., Premature nonsense codons decrease the stability of phytohemagglutinin mRNA in a position-dependent manner, Plant J., 1996, 10, 415-424. 

A bromoacetylated derivative of the nonsense codon UpGpA has been synthesized and used to label ribosomal proteins.103... 

The stem-loop structure in the noncoding 3 region of selenoprotein mRNAs has also been termed a SECTS element in mammals although it has a different overall structure. ° In silica analysis of the human genome sequence, using this consensus SECTS element along with the presence of the characteristic UGA codon within an exon, has led to the discovery of several new selenoproteins, including a selenium-dependent methionine sulfoxide reductase. It has been shown that a specific complex exists for selenoprotein synthesis that shuttles between the nucleus and the cytosol. This possibly protects the preformed complex for nonsense-mediated decay to allow for more efficient selenoprotein synthesis. The specific tRNA needed for selenocysteine... 

Figure 10 Alteration of the genetic code for incorporation of non-natural amino acids, (a) In nonsense suppression, the stop codon UAG is decoded by a non-natural tRNA with the anticodon CUA. In vivo decoding of the UAG codon by this tRNA is in competition with termination of protein synthesis by release factor 1 (RFl). Purified in vitro translation systems allow omission of RF1 from the reaction mixture, (b) A new codon-anticodon pair can be created using four-base codons such as GGGU. Crystal structures of these codon-anticodon complexes in the ribosomal decoding center revealed that the C in the third anticodon position interacts with both the third and fourth codon position (purple line) while the extra A in the anticodon loop does not contact the codon.(c) Non-natural base pairs also allow creation of new codon-anticodon pairs. Shown here is the interaction of the base Y with either base X or (hydrogen bonds are indicated by red dashes).

Nonsense new codon is stop codon Shorter than normal usually nonfunctional... 

When any of the three stop (termination or nonsense) codons moves into the A site, peptidyl transferase (with the help of release fector) hydrolyzes the completed protein from the final tRNA in the P site. The mRNA, ribosome, tRNA, and factors can aU be reused for additional protein synthesis,... 

A nonsense mutation (choice D) is a single nucleotide substitution that produces a stop codon and thus truncation of the polypeptide. Therefore, it typically alters more than a single amino acid. 

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