Amino Nonsense codon

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. 

Termination ("stop" or "nonsense ) codons Three of the codons, UAG, UGA, and UAA, do not code for amino acids, bit rather are termination codons. When one of these codons appears in an mRNA sequence, it signals that synthesis of the peptide chain coded for by that mRNA is completed. 

In addition to those codons assigned to specific amino acids, three are designated as chain termination codons UAA, UAG, and UGA. These are frequently referred to as "nonsense" codons. The termination codons UAA and UAG are also known as ochre and amber, respectively, although these names have no scientific significance.41 The codons AUG (methionine)... 

Nonsense mutation. A change in the base sequence that converts a sense codon (one that specifies an amino acid) to one that specifies a stop (a nonsense codon). There are three nonsense codons. 

The loss or gain of one or two base pairs in a gene causes an incorrect reading of the DNA and is known as a frameshift mutation. This is illustrated in Figure 8.2, which shows the insertion of a single base pair into a gene. It is seen that subsequent codons are changed, which almost always means that there are nonsense codons that specify no amino acid. So either no protein or a useless protein is likely to result. 

CODON Three contiguous nucleotides in mRNA that specify the amino acid to be inserted at a specific position in a polypeptide during translation of the 64 possible codons that are formed by the four types of nucleotides in SNA (nucleotides of adenine, guanine, cytosine, and uracil), 61 specify an amino acid and three (nonsense codons) specify no amino acid, but rather serve as termination signals in protein synthesis. 

NONSENSE CODON A codon that does not specify any amino acid in protein synthesis, but instead specifies termination of a polypeptide chain the nonsense codons are UAA, UAG, and UGA. 

NONSENSE MUTATION A mutation that changes a codon for a particular amino acid into a nonsense codon. (See also MISSENSE MUTATION)... 

In another study, proteins containing both mono- and diglycosylated amino acids, including glycosylated serine and tyrosine moieties, have been obtained by the suppression of nonsense codons in a cell-free expression system through the application of misacylated suppressor tRNAs activated with glycosylated serine and tyrosine derivatives (O Fig. 3) [32]. 

The small insertion/deletion mutations account for about 23% of the nucleic acid sequence alterations that cause disease. An insertion refers to the presence of extra bases while deletion implies the absence of certain bases in comparison to a reference sequence. Insertion and deletion mutations often result in a shift of the codon reading frame, resulting in altered amino acid sequence downstream of the mutation—commonly followed by chain termination from a nonsense codon. Indels are deletions followed by insertions (e.g., replacement of AGGTC by TG). 

Furthermore, some base exchanges result in so-called silent mutations which do not cause amino acid exchanges in the enzyme. In addition, nonsense codons also occur which usually result in termination of translation. 

C), the mutation is cdX cd temperature-sensitive (Ts) mutation. Sometimes no amino acid corresponds to a new base sequence (Chapter 25) in that case, termination of synthesis of the protein occurs at that point, and the mutation is called a chain termination mutation or nonsense mutation. These mutations generate one of the three nonsense codons UAA, UAG, or UGA (Chapter 25). 

The answer is b. (Murray, pp 452-467. Scriver, pp 3-45. Sack, pp 245-257. Wilson, pp 151-180.) The replacement of the codon UAG with UAA would be a silent mutation since both codons are stop signals. Thus, transcription would cease when either triplet was reached. There are three termination codons in mRNA UAG, UAA, and UGA. These are the only codons that do not specify an amino acid. A missense or a substitution mutation is the converting of a codon specifying one amino acid to another codon specifying a different amino acid. A nonsense mutation converts an amino acid codon to a termination codon. A suppression counteracts the effects of another mutation at another codon. The addition or deletion of nucleotides results in a frame-shift mutation. 

The convention is that one of the codons or triplets is used as a start signal for protein synthesis, and three for a stop signal. The start signal is AUG, which also serves as the codon for the genetic instructions to produce methionine. The stop signals are UAA, UAG, and UGA, which are the instructions for no known amino acids, and are therefore designated as nonsense codons. 

There are three types of codons. There is an initiation codon, AUG, which signifies the initial amino acid (and also codes for methionine residues in internal positions) in the protein. There are 61 codons, including AUG, that designate individual amino acids. The remaining three codons (UAA, UAG, and UGA) are termination codons (also called stop codons or nonsense codons), which do not code for amino acids, but sigual the eud of the mRNA message and provide the stop signal for protein synthesis. 

A completely autonomous bacterium with a 21 amino acid genetic code was engineered. The bacterium can generate p-aminophenylalanine from basic carbon sources and incorporate this amino acid into proteins in response to the amber nonsense codon [132]. 

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