The meiotic behavior of two graded series of deletion mutations in the ADE8 gene in Saccharomyces cerevisiae was analyzed to investigate the molecular basis of meiotic recombination. Postmeiotic segregation (PMS) was observed for a subset of the deletion heterozygosities, including deletions of 38 to 93 base pairs. There was no clear relationship between deletion length and PMS frequency. A common sequence characterized the novel joint region in the alleles which displayed PMS. This sequence is related to repeated sequences recently identified in association with recombination hotspots in the human and mouse genomes. We propose that these particular deletion heterozygosities escape heteroduplex DNA repair because of fortulitous homology to a binding site for a protein.Meiotic recombination is the fundamental process which is responsible for the reassortment of genetic information between paired homologous chromosomes in zygotic organisms. The physical recombination event takes place after premeiotic DNA replication but before the first nuclear division (1). At that time, each locus is represented on eight DNA strands, i.e., two double-stranded copies of each homolog. Ordinarily, interaction between homologs results in a detectable exchange of information. This exchange can involve one or two strands of DNA and can be reciprocal or nonreciprocal. A nonreciprocal exchange is termed a gene conversion. Because reciprocal and nonreciprocal exchanges are nonrandomly associated in meiosis, gene conversion can be regarded as a signature of the recombination process (5). We study recombination in Saccharomyces cerevisiae because of the powerful confluence of available molecular and genetic technologies. In particular, tetrad dissection of sporulated, appropriately marked diploid strains, followed by replica plating of the resulting four undisturbed ascosporal colonies, allows us to visualize the ultimate fates of each of the eight DNA strands which were present before the meiotic divisions. In addition, replacement of resident alleles by in vitro-modified sequences is now a routine procedure (2).Postmeiotic segregation (PMS) is detected phenotypically as the sectoring of a single heterozygous marker in a haploid ascosporal colony and represents marker segregation at the first postmeiotic mitosis. Presumably, such segregation results when the spore contains uncorrected heteroduplex DNA which encompasses the heterozygosity. PMS at ADE8 can be visualized as red and white half sectors caused by the interaction of ADE2 and ADE8. PMS in yeast was first observed by Esposito in an ade8-181ADE8 heterozygote (3). In a previous analysis, it was found that PMS frequency can be correlated with the potential heteroduplex DNA mismatch (16) generated by the interacting alleles. Furthermore, among those analyzed, the mutant allele displaying the highest PMS frequency (ade8-18) was determined to harbor a 38-base-pair (bp) deletion. Since previous studies showed that large deletions and frameshift mutations do not display appreciable PMS l...
P J P O = dimensionless. temperature r. for nonreacting system, = dimensionless velocity, U (r,/p")l'z = dimensionless distance, ( r J p n ) d P 0 Y Subscripts a n d Superscripts b 0 = subscript denoting bulk mean = subscript denoting wall con-conditions ditions -= molal basis = value in reacting systemThe selectivity of a polar solvent is expressed in terms of an approximate theory of solutions. The theoretical results are insufficiently precise for the accurate prediction of activity coefficients, but the analysis shows that i n the absence of chemical effects selectivity depends primarily on the difference i n molar volumes of the hydrocarbons to be separated and on the polar energy density of the solvent. The effectiveness of o solvent i s related to its polarity (which should be large) and t o i t s molecular size (which should be small). In cases where chemical effects are important or where the molar volumes of the hydrocarbons t o be separated are only slightly different, selectivity olso depends on the relative ability of the hydrocarbons i n acting as electron donors and on the ability of the solvent t o act as an electron acceptor in forming acid-base complexes. The theoretical conclusions, which are based on modern thermodynamics and on the theory of intermolecular forces, are i n agreement with experimental observations. The results obtained i n this work provide theoretical criteria for the selection of an optimum solvent for a given separation and phenomena related to extractive distillation.
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