Apparent Epigenetic Meiotic Double-Strand-Break Disparity in Saccharomyces cerevisiae: A Meta-Analysis.

Previously published, and some unpublished, tetrad data from budding yeast (Saccharomyces cerevisiae) are analyzed for disparity in gene conversion, in which one allele is more often favored than the other (conversion disparity). One such disparity, characteristic of a bias in the frequencies of meiotic double-strand DNA breaks at the hotspot near the His4 locus, is found in diploids that undergo meiosis soon after their formation, but not in diploids that have been cloned and frozen. Altered meiotic DNA breakability associated with altered metabolism-related chromatin states has been previously reported.

Amber mutants of bacteriophage T4D: their isolation and genetic characterization.

We have isolated a large number of mutants of bacteriophage T4D that are unable to form plaques on strain B of Escherichia coli, but are able to grow (nearly) normally on some other strains of E. coli, in particular strain CR63. These mutants, designated amber (am), have been characterized by complementation tests, by genetic crosses, and by their response to chemical mutagens.

A two-pathway analysis of meiotic crossing over and gene conversion in Saccharomyces cerevisiae.

Several apparently paradoxical observations regarding meiotic crossing over and gene conversion are readily resolved in a framework that recognizes the existence of two recombination pathways that differ in mismatch repair, structures of intermediates, crossover interference, and the generation of noncrossovers. One manifestation of these differences is that simultaneous gene conversion on both sides of a recombination-initiating DNA double-strand break ("two-sidedness") characterizes only one of the two pathways and is promoted by mismatch repair. Data from previous work are analyzed quantitatively within this framework, and a molecular model for meiotic double-strand break repair based on the concept of sliding D-loops is offered as an efficient scheme for visualizing the salient results from studies of crossing over and gene conversion, the molecular structures of recombination intermediates, and the biochemical competencies of the proteins involved.

Methods for analysis of crossover interference in Saccharomyces cerevisiae.

Interest in crossover interference in yeast has been spurred by the discovery and characterization of mutants that alter it as well as by the development and testing of models to explain it. This chapter describes methods for detecting and for measuring interference, with emphasis on those that exploit the ability to examine all four products of individual acts of meiosis.

On Spo16 and the coefficient of coincidence.

spo16 mutants in yeast were reported to have reduced map lengths, a high frequency of nondisjunction in the first meiotic division, and essentially unchanged coefficients of coincidence. Were all crossing over in yeast subject to interference, such data would suggest that the "designation" of recombination events to become crossovers is separable from the "implementation" of that crossing over. In the presence of coexisting interference and noninterference phases of crossing over, however, lack of change in the coefficient of coincidence may show only that spo16 reduces crossing over in the two phases by a similar factor.

On the "NPD ratio" as a test for crossover interference.

The "NPD ratio," widely used by yeast geneticists, is of limited applicability and is prone to falsely indicate significant crossover interference in a chi-square test. A simple, better chi-square test for interference in two-factor crosses is described.

Reduced mismatch repair of heteroduplexes reveals "non"-interfering crossing over in wild-type Saccharomyces cerevisiae.

Using small palindromes to monitor meiotic double-strand-break-repair (DSBr) events, we demonstrate that two distinct classes of crossovers occur during meiosis in wild-type yeast. We found that crossovers accompanying 5:3 segregation of a palindrome show no conventional (i.e.

Crossover interference on nucleolus organizing region-bearing chromosomes in Arabidopsis.

In most eukaryotes, crossovers are not independently distributed along the length of a chromosome. Instead, they appear to avoid close proximity to one another--a phenomenon known as crossover interference. Previously, for three of the five Arabidopsis chromosomes, we measured the strength of interference and suggested a model wherein some crossovers experience interference while others do not.

Gene conversion and crossing over along the 405-kb left arm of Saccharomyces cerevisiae chromosome VII.

Gene conversions and crossing over were analyzed along 10 intervals in a 405-kb region comprising nearly all of the left arm of chromosome VII in Saccharomyces cerevisiae. Crossover interference was detected in all intervals as measured by a reduced number of nonparental ditypes. We have evaluated interference between crossovers in adjacent intervals by methods that retain the information contained in tetrads as opposed to single segregants.

Does crossover interference count in Saccharomyces cerevisiae?

We previously proposed a "counting model" for meiotic crossover interference, in which double-strand breaks occur independently and a fixed number of noncrossovers occur between neighboring crossovers. Whereas in some organisms (group I) this simple model alone describes the crossover distribution, in other organisms (group II) an additional assumption--that some crossovers lack interference--improves the fit. Other differences exist between the groups: Group II needs double-strand breaks and some repair functions to achieve synapsis, while repair in group I generally occurs after synapsis is achieved; group II, but not group I, has recombination proteins Dmc1, Mnd1, and Hop2.

Gene products encoded in the ninR region of phage lambda participate in Red-mediated recombination.

Background: The ninR region of phage lambda contains two recombination genes, orf (ninB) and rap (ninG), that were previously shown to have roles when the RecF and RecBCD recombination pathways of E. coli, respectively, operate on phage lambda.

Results: When lambda DNA replication is blocked, recombination is focused at the termini of the virion chromosome.