Mutation conditionally lethal

Genetic Control. Manipulation of the mechanisms of inheritance of the insect pest populations has occurred most successhiUy through the mass release of steri1i2ed males, but a variety of other techniques have been studied, including the environmental use of chemostetilants and the mass introduction of deleterious mutations, eg, conditional lethals and chromosomal translocations (58 ndash 60) (see Genetic engineering). 

Temperature-sensitive mutations are those which allow a virus to replicate at one temperature and not at another, due to a mutational alteration in a virus protein that renders the protein unstable at moderately high temperatures. For instance, temperature-sensitive mutants are known in which the phage will not be replicated in the host at 43 °C but will at 25 °C, although the host functions at both temperatures. Such mutations are called conditionally lethal, since the virus is unable to reproduce at the higher temperature, but replicates at the lower temperature. 

Sommers, C.H., Miller, E.J., Dujon, B., Prakash, S. and Prakash, L. (1995) Conditional lethality of null mutations in RTH1 that encodes the yeast counterpart of a mammalian 5- to 3 -exonuclease required for lagging strand DNA synthesis in reconstituted systems. J. Biol. Chem., 270, 4193 1196. 

T Inada, DL Court, K Ito, Y Nakamura. Conditionally lethal amber mutations in the leader peptidase gene of Escherichia coli. J Bacteriol 171 585-587, 1989. 

Nutritional auxotrophs can be described as conditionally lethal mutants they survive only if the medium is supplemented with the nutrient, whose S5mthesis depends upon the missing enzyme. Other kinds of conditional lethal mutations permit study of almost every gene in an organism. For example, temperature-sensitive (fs) mutants grow perfectly well at a low temperature, e.g., 25°C, but do not grow at a higher temperature, e.g., Many... 

Bjornsti MA, Wang JC. Expression of yeast DNA topoisomerase I can complement a conditional-lethal DNA topoisomerase I mutation in Escherichia coli. Proc Natl Acad Sci USA 1987 84 8971-8975. 

The conversion of nicotinic acid mononucleotide to NAD proceeds in two metabolic steps, with nicotinic acid dinucleotide (NaAD) as an intermediate (4) there are no alternative metabolic steps for the synthesis of NAD from NaMN. Thus, the genes that control these two metabolic steps should be essential for viability, and conditional-lethal mutations should be recovered. 

With respect to cell viability, a mutation can be lethal, conditionally lethal, or nonlethal. Conditionally lethal mutations (e.g., ts mutations) are lethal under one set of conditions nonpermissive) whereas they exhibit no critical deficiency for cell viability under alternative or permissive conditions. 

Phenotypic analysis of the mutants suggests that the gene produa plays a role in multiple functions such as the repair of damaged or alkylated DNAs. However, none of the mutations at the single xth gene are conditionally lethal. The xth mutants are extremely sensitive to hydrogen peroxide and UV irradiations (37,38) and are unable to express a heat-shock response (39). 

Cystic fibrosis (CF) is the most common potentially lethal autosomal recessive disease among Caucasians. The incidence is estimated to be approximately 1 in 2000 births (Bl). Since it is inherited as an autosomal recessive condition, screening to identify couples at risk has been suggested. However, CF screening is complicated because many mutations of the CF gene exist. Thus, the feasibility of screening a population for carriers of cystic fibrosis gene mutations is primarily dependent upon the frequency of the common mutation in that population. 

A 30-year-old man is phenotypically normal, but two of his siblings died from infantile Tay-Sachs disease, an autosomal recessive condition that is lethal by the age of five. What is the risk that this man is a heterozygous carrier of the disease-causing mutation ... 

In a test for sex-linked recessive lethal mutations in Drosophila, Wild et al. 8 found no evidence of mutagenicity of CS. More than 9,000 chromosomes were tested, and the frequencies of mutations in the treated groups did not differ from those in the concurrent negative controls or the historical negative controls. The available information on the toxicity of CS under the treatment conditions is minimal. The actual dosages received by the flies are also uncertain, particularly because CS breaks down rapidly in water. Nevertheless, the available data give no indication of mutagenicity of CS in Drosophila. 

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