Termination triplet

Abbreviations used for purine and pyrimidine bases on ribonucleotides U, uridine C, cytosine A, adenine G, guanine. Stop och) refers to the ochre termination triplet, Stop (amb) refers to the amber termination triplet, and Stop (opal) refers to the opal termination triplet. 

There are actually three codons that function in this way. They are UAA, UGA, and UAG and are referred to as the chain termination triplets, or stop codons (see Table 9-1). 

The basic structure of collagen consists of the protofibril—a triple helix consisting of two identical a chains, as they are called, and one different chain. The a i chain of calf or rat hide consists of 1 052 a-amino acid residues, of which 1 001 are triplets of the general structure (gly-X-Y). Here, X may be proline, leucine, phenyl alanine, glutamic acid, and Y is mostly hydroxy-proline or arginine. Collagen is the only protein to contain hydroxyproline. This imino acid residue, however, is only formed after the protein biosynthesis. The triplets are, in turn, joined in sequences of polar and apolar regions. Telopeptides, peptide structures without triplet structure, occur at the N- and C-terminal triplet structures. The telopeptides are rich in lysine and they account for the intra- and intermolecular covalent cross-links. 

Both ERAS contain 7 termination triplets within the 5 terminal 30 nucleotides, encompassing all possible reading frames. G hree further nonsense triplets occur up to position -125 in EMCV ERA, and at least 26 of the 3 terminal nucleotides cannot therefore be translated, and therefore 5 nonsense triplets within the last 20 nucleotides are not normally used as termination codons. 

Only 61 triplets code for amino acids. The remaining three (UAA, UAG, and UGA) are signals for chain termination they signal to the protein-synthesizing machinery of the cell that the primary sequence of the protein is complete. The three chain termination triplets are indicated in Table 20.4 by Stop. ... 

Other variants of potential interest are those with internal deletion and reinitiation of normal synthesis at a methionine residue, position 252 (Table 1.8-III, Figure 1.18). Codon AUG functions as an initiation signal in protein biosynthesis in prokaryotes and eukaryotes . When AUG occurs internally, it codes for insertion of methionine. Internal AUG initiation sites have been described, but only in the presence of a nearby terminator triplet Whether or not the same mechanism is operating in this type of variant remains to be elucidated. 

The other important sequence is the CCA triplet at the 3 end The ammo acid that IS to be transferred is attached through an ester linkage to the terminal 3 oxygen of this sequence All tRNAs have a CCA sequence at their 3 end... 

The specific ribonucleotide sequence in mRNA forms a message that determines the order in which amino acid residues are to be joined. Each "word," or codon, along the mRNA chain consists of a sequence of three ribonucleotides that is specific for a given amino add. For example, the series UUC on mRNA is a codon directing incorporation of the amino acid phenylalanine into the growing protein. Of the 43 = 64 possible triplets of the four bases in RNA, 61 code for specific amino acids and 3 code for chain termination, fable 28.1 shows the meaning of each codon. 

An opening frame contains a series of codons (base triplets) coding for amino acids without any termination codons. There are six potential reading frames of an unidentified sequence. 

The terms first, second, and third nucleotide refer to the individual nucleotides of a triplet codon. U, uridine nucleotide C, cytosine nucleotide A, adenine nucleotide G, guanine nucleotide Term, chain terminator codon. AUG, which codes for Met, serves as the initiator codon in mammalian cells and encodes for internal methionines in a protein. (Abbreviations of amino acids are explained in Chapter 3.)... 

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

The 64 triplet codons are listed in the 5 —>3 direction in which they are read. The three termination (term) ... 

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