Frameshift changes

Base substitutions and frameshift changes occur spontaneously and can be induced by radiation and chemical mutagens. It is apparent that the molecular mechanisms resulting in these changes are different in each case, but the potential hazards associated with mutagens capable of inducing the different types of mutation are equivalent. 

Resistance to certain antibiotics can arise as a consequence of mutations to chromosomal genes because of changes in the DNA sequence. Mutations can occin due to single base pair changes. Transitions involve the substitution of one purine (A or G) for another and therefore one pyrimidine (C or T) for another. Transversions involve a change from a pyrimidine to a purine and vice versa. Frameshift mutations occin when one or... 

More extensive changes in the DNA sequence (often referred to as macrolesions) can also occur. Deletions result in the loss of part of the DNA sequence. Insertions add extra base pairs to a gene. Transversions occur when a segment ofthe DNA is reversed and duplications occur when a segment ofthe DNA is repeated. Some ofthese changes also result in frameshifts. 

Frameshift suppression is also possible. This can be achieved by a second mutation in a tRNA gene such that the anticodon of a tRNA molecule consists of 4 bases rather than 3, for example, an extra C residue in the CCC anticodon sequence of a glycine tRNA gene. This change will allow correction of a +1 frameshift involving the GGG codon for glycine (Bossi, 1985). 

Genetic Make up of Tester Strains. The most widely used strains are those developed by Bruce Ames and colleagues which are mutant derivatives of the organism Salmonella typhimurium. Each strain carries one of a number of mutations in the operon coding for histidine biosynthesis. In each case the mutation can be reverted either by base-change or by frameshift mutations. The genotype of the commonly used strains is shown in Table 6.6. 

Mutations are often classified according to the effect they have on the structure of the gene s protein product. This change in protein structure can be predicted using the genetic code table in conjunction with the base sequence of DNA or mRNA. A variety of such mutations is listed in Table 1-4-1. Point mutations and frameshifts are illustrated in more detail in Figure 1-4-2. 

The effects of small changes to the base code such as base transitions and frameshift mutations are illustrated in Table 6.1. It can be seen that the consequences in translation vary with the particular change. 

Changing Horses in Midstream Translational Frameshifting and mRNA Editing... 

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. 

Mutations in CYP2D6 (25) may lead to three kinds of functional changes absence, decrease, or increase of enzyme activity. Lack of enzyme activity indicates in most cases a frameshift mutation or a splicing defect, but there is also a case of totally missing enzyme formation. Decreases of enzyme activity indicate a change of enzyme structure these can be clinically treacherous since the decrease may differ in severity for different sustrates. Enhanced enzyme activity indicates usually gene duplication, sometimes multiplication a case of 13-fold multiplication has been observed. Everyone of these enzyme changes may differ between human populations. 

Frameshift Mutation. Deletion or insertion of a base or bases that alters the reading frame of the gene. In contrast to base substitution mutations where the mutation often has no consequence to the gene function or has only a partial effect on the gene function, a frameshift mutation completely changes the amino acid sequence downstream of the mutation site within a proteincoding gene. Thus, frameshift mutation is a severe type of mutation, which results in inactivation of the protein product most of the time. 

Ames developed strains of bacteria that had carefully selected lethal mutations. In a test system the bacteria could survive only when its mutation had been corrected by experiencing another mutation caused by the tested material. This correction could be accomplished by causing a point mutation or frameshift mutations . Point mutations are base-pair substitutions, that is, a base change in DNA of at least one DNA base pair. In a reverse mutation test, this change in base pairs may occur at the site of the original mutation, or at a secondary site in the bacterial genome. Frameshift mutations are the addition or deletion of one or more base pairs in the DNA. Since amino acids are encoded by triplets of base pairs in sequence, any addition or deletion of 1 or 2 base pairs will dramatically alter the expressed protein from that point on. The Ames system employs strains of Salmonella typhimurium and Escherichia coli that require amino acids (histidine or tryptophan, respectively) to detect such reverse point and frameshift mutations. The reverse mutation allows the S. typhimurium or E. coli strains to restore the functional capability of the bacteria to be able to synthesize the specific amino acid on their own, independent of amino acid content in the medium. 

In most countries, about one third of the mutations that are identified wfll not have been reported previously in AIP and may represent rare polymorphisms rather than disease-specific mutations. Criteria that suggest that such novel mutations cause disease include production of a frameshift or stop codon, the absence of any other sequence abnormality in the gene, segregation with disease, and nonconservative change of an amino acid residue that is conserved between species and/or known to have a functional role in catalysis. Mutations of consensus bases in splice sites are also likely to be disease specific, but ideally all putative splicing defects should be confirmed by analysis of mRNA. Proof that a missense mutation causes disease may require expression and characterization of the mutant enzyme in a prokaryotic or eukaryotic vector. 

D. Since the [3-chain is decreased with respect to the a-chain, it is most likely that there is a mutation that decreases the expression of the [3-chain gene, in which a mutation in the promoter region could result. A point mutation in the P-chain leading to an amino acid substitution could lead to changes in electrophoretic mobility but would not alter the levels of expression. A frameshift mutation in the P-chain would result in decreased P-chain on the electrophoregram. 

Translational frameshifting. Certain mRNAs appear to contain structural information that, if activated, results in a +1 or -1 change in reading frame. This translational frameshifting, which has been most often observed in cells infected by retroviruses, allows more than one polypeptide to be synthesized from a single mRNA. 

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