Triplet nonsense

A suppressor of frame-shift mutations in Salmonella is a tRNA containing at the anticodon position the nucleotide quartet CCCC instead of the usual CCC triplet anticodon.442 443 It has eight unpaired bases in the anticodon loop instead of the usual seven. Other frame-shift repressor tRNAs have been identified in E. cob, 444 Salmonella, and yeast.445 Not all suppressor genes encode tRNAs. For example, a UGA suppressor from E. coli is a mutant 16S rRNA from which C1054 has been deleted.446 A general nonsense suppressor in yeast is homologous to yeast elongation factor EF-la as well as to E. coli EF-Tu.447... 

There are actually three codons that function in this way. They are UAA, UGA, and UAG, and are referred to as the chain termination (CT) triplets (see Table 17.1), stop codons, or nonsense codons. 

Messenger RNA (mRNA) molecules specify only one polypeptide chain Many triplets can be nonsense triplets... 

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. 

Genetic code The set of codons in mRNA, specifying each of 61 possible tRNA anticodons and 3 nonsense (STOP) codons it is based on 43 triplet combinations. 

Codon established by synthesis, but the triplet is not effective in tRNA. [ ], Start condon (see text), end. Nonsense codon (termination). 

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. 

Some polar mutations of the lac operon have been found to be suppressed by other genes which are known to act as amber, ochre, or UGA suppressors. When z nonsense mutants are suppressed, an amino acid is inserted at the point of the mutation and the mutated enzyme activity is restored. The inserted amino acid has been found to be related to the class of suppressor employed. Suppressor genes produce mutated species of tRNA s which recognize nonsense codons, and insert amino acids at that point. For example the tyrosine tRNA of suppressor III recognizes the nonsense triplet UAG and inserts tyrosine on the growing peptide chain, thereby competing with the chain release reaction. 

A single base change may produce yet another effect—it may create one of the three triplets which do not specify any amino acid. It has been shown that when the protein-synthesizing machinery encounters one of these nonsense codons, growth of the polypeptide is terminated [119-121]. As they cause the production of only a fragment of the intended protein, mutations of the nonsense type almost always destroy a gene s function. 

These procedures have permitted researchers to determine the base composition and the base sequence of the codons for a large number of amino acids and to establish that the code is degenerate, universal, operates in vivo, and is not overlapping. The accepted code for each amino acid is presented in Table 2-4. There are 64 sequence combinations of trinucleotides when four different bases are used to build the triplet. A brief look at Table 2-4 shows that 60 of the 64 combinations are involved in amino acid coding. Two sequences have no known coding properties (CUA and CUG) and are therefore called nonsense codons. The intercalation of such a triplet into the sequence of messenger RNA is responsible for nonsense mutation. UAA and UAG are now known to be involved in punctuation. 

A number of triplets are referred to as nonsense codons because they are not known to code for any specific amino acid. When bacterial strains were developed in which mutations leading to the appearance of such triplets in their messenger were produced, 3 of the so-called nonsense codons, UAG, UGA (amber mutation), and UAA (ocre mutation) were found to act as punctuation codons for termination of the chain. 

Garen, A. Sense and nonsense in the genetic code, 3 exceptional triplets can serve as both chain-terminating signals and amino acid coding. Science 160, 149-159(1968)... 

Shine, J., and Dalgarno, L. (1974). The 3 -terminal sequence of Escherichia coli 16S ribosomal RNA complementarity to nonsense triplets and ribosome binding sites. Proceedings of the National Academy of Sciences of the United States of America 71, 1342-1346. 

The boundaries between cistrons (zones for synthesis of individual proteins) within polycistron templates providing for synthesis of polypeptides of a particular length are satisfactorily explained (on the basis of theoretical and experimental data) by the hypothesis of the existence of nonsense triplets, i.e., codons to which there are no anticodons in the sRNA fraction and which do not code the position of any amino acid (Brenner et al., 1965). Of the 64 possible three-nucleotide codons, even allowing for degeneracy of the code, not all determine the position of an amino acid. 

The assumption is that each codon consists of three adjacent bases, although this triplet code has not yet been proved rigorously. Another requirement is that the start for reading off a series of triplets must be signaled in an as yet unknown manner. As a result of mutation, either by deletion of a base or by insertion of a new one, the sequence is misarranged and so many nonsense triplets are created that no protein is formed for a given gene. Such a mutation is called a deletion mutation. 

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