Codons, stop

Analysis of tlie global statistics of protein sequences has recently allowed light to be shed on anotlier puzzle, tliat of tlie origin of extant sequences [170]. One proposition is tliat proteins evolved from random amino acid chains, which predict tliat tlieir length distribution is a combination of the exponentially distributed random variable giving tlie intervals between start and stop codons, and tlie probability tliat a given sequence can fold up to fonii a compact... 

In addition to codons for ammo acids there are start and stop codons Protein biosynthesis begins at a start codon and ends at a stop codon of mRNA The start codon IS the nucleotide triplet AUG which is also the codon for methionine The stop codons are UAA UAG and UGA... 

When the cell requires instructions for protein production, part of the code on DNA, starting at an initiator and ending at a stop codon, is converted into a more mobile form by transferring the DNA code into a matching RNA code on a messenger ribonucleic acid (mRNA), a process known as transcription. The decoding, or translation, of mRNA then takes place by special transfer ribonucleic acids (tRNA), which recognize individual codons as amino acids. The sequence of amino acids is assembled into a protein (see Proteins section). In summary, the codes on DNA... 

Figure 38-9. Diagrammatic representation of the termination process of protein synthesis. The peptidyl-tRNAand aminoacyl-tRNA sites are indicated as P site and A site, respectively. The termination (stop) codon is indicated by the three vertical bars. Releasing factor RF1 binds to the stop codon. Releasing factor RF3, with bound GTP, binds to RFl. Flydrolysisofthe peptidyl-tRNA complex is shown by the entry of HjO. N and C indicate the amino and carboxyl terminal amino acids, respectively, and illustrate the polarity of protein synthesis.

Termination Occurs When a Stop Codon Is Recognized (Figure 38-9)... 

The complete sequence of the pgaX gene has been determined. Seven introns are present in the gene which were all confirmed by cDNA sequencing. The introns are between 50 and 58 nt long, have fungal consensus splice sequences and all but one have in frame stop codons. 

A surprising and unique feature of the human VN genome is not its small size, but that it exists at all. Of the eight loci located, seven are inoperative, due to frame-shifts and/or stop codons the one potentially functional gene, named V1RL1, was found in all individuals from a... 

Vidal R, Fragione B, Rostagno A, Mead S, Revesz T, Plant G, Ghiso J. A stop-codon mutation in the BRI gene associated with familial British dementia. Nature 1999 399 776-781. 

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