Stop code: UAG, UAA, UGA are stop codons. In 1964, Yanofsky speculated on the presence of nonsense codons when studying E. coli tryptophan synthase A protein. His speculation is from two different perspectives: First, the mRNA encoding trpA also encodes trpB, trpC, trpD and trpE. That is, an mRNA molecule can be used as a template for different polypeptides, so it is possible to stop at a certain position (the junction of two peptides) during translation, and then translate from the next new starting point, so that each peptide can be separated Instead of producing a very long peptide chain. This means the presence of a stop codon. Another angle is that he found that the mutant strain of E. coliTrp- cannot synthesize a complete tryptophan synthase protein, but continues to It can be mutagenized to get back mutations. There are two kinds of back mutations, one is that an individual has changed, and the other is a complete reversion, without any change in amino acid composition, which shows that E.coliTrp- cannot be any As a result of frameshift mutations, such mutations are likely to carry nonsense codons that prevent synthesis.
In 1962, when Benzer and his student S. Champe studied the mutation of T4râ…¡, it was found that the wild-type T4râ…¡ had two cistrons râ…¡A and râ…¡B, which transcribed a polycistronic mRNA together, but translated into two separate proteins. A and B. When a deletion mutation occurs, one of the mutations is rl589, which proves to be caused by the deletion. The deleted region contains most of the right side of the rIIA gene and a small part of the left side of rIIB. Complementary experiments show that the product of rl589 is a polypeptide, but no protein A activity, but protein B activity. Benzer believes that this deletion may cause the mRNA to lose the "termination" of protein A synthesis and the "start" of "B" protein synthesis, so a long peptide chain is produced by reading along an mRNA during translation. In 1964, Brenner and his colleagues obtained the amber mutation of the gene encoding the head protein of T4 bacteriophage, and carried out detailed mapping, and isolated and studied various mutant polypeptides. The mutant peptide chain is shorter than that of the wild type, so it can be speculated that the amber mutation may produce a stop codon, which stops the synthesis of the peptide in the middle; the closer the mutation site is to the left end of the gene, the shorter the peptide chain produced, The closer to the right end, the closer to the wild type, from which we can speculate that the translation process is to read from the 5 'end of the mRNA. The synthesis of the peptide chain extends from the N-terminus to the C-terminus. Since 80% of the head protein is composed of newly synthesized protein. Therefore, 10 minutes after infecting various mutant and wild-type T4 bacteriophages with E. coli, the 14C-labeled amino acids were added to the culture medium. After a period of time, the protein and head protein were extracted from the infected E. coli. Can be identified by the 14C mark. Their experimental method is not to sequence the products of various mutants, but to first treat the wild-type head protein with trypsin and chymotrypsin. The extremely complex mixture produced after digestion can be separated and identified by electrophoresis. There are 8 characteristic head protein fragments, which are Cys, T7C (His), C12b (Tyr), T6 (Trp), T2a (Pro), T2 (Trp), C2 (Tyr) and C5 (His ) Snippet. Then it was determined that each T4 head protein mutant product contained more than a few peptides to rank. The result of sorting is consistent with the sequence of fine mapping, which not only shows the collinear relationship between gene and protein, but also proves the existence of nonsense mutations in the mutant head protein gene, and its position should be at the end of various mutation products .
Until 1965, Weigert, M. and Ggaren, A. The substitution of amino acids at the tryptophan site in the alkaline phosphatase gene proved that the base composition of the nonsense codon in E. coli revealed mutant genes of amber and ochre These are the stop codons UAG and UAA. At that time, 61 of the 64 passwords had been deciphered, leaving only UAA, UAG and UGA to be determined. Garen et al. Adopted a similar strategy to Brenner in order to identify nonsense codons. They isolated a large number of revertant mutants from a nonsense mutant strain in the alkaline phosphatase gene (phoA) of E.coli, and then explored each nonsense mutation in the peptide equivalent to the recovered nonsense What exactly is the amino acid at the codon position? It can be seen that the nonsense codon is generated from the codon at the tryptophan site of the gene. In back mutations, the nonsense codons become the corresponding codons of Trp, Ser, Tyr, Leu, Glu, Gln, and Lys. Only the UGG of Trp becomes UAG, and on this basis it is back mutated to 7 amino acids, so the codon for the nonsense mutation produced by Trp is UAG. In 1967, Brennr and Crick proved that UGA was the third nonsense codon. According to the three nicknames for nonsense mutations, the three stop codons UAA are called ochre codons (corresponding to ochre mutations); UAG are called amber codons (corresponding to amber mutations); UGA is called opal codons (opal) Corresponds to the opal mutation).
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