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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. Interference was seen between intervals when the distance in the region adjacent to a crossover was Ͻ‫53ف‬ cM (90 kb). At the met13 locus, which exhibits ‫%9ف‬ gene conversions, those gene conversions accompanied by crossing over exerted interference in exchanges in an adjacent interval, whereas met13 gene conversions without an accompanying exchange did not show interference. The pattern of exchanges along this chromosome arm can be represented by a counting model in which there are three nonexchange events between adjacent exchanges; however, maximum-likelihood analysis suggests that ‫%21-8ف‬ of the crossovers on chromosome VII arise by a separate, noninterfering mechanism. I N meiosis, recombinational repair of double-strand independence of crossing over in two monitored intervals. Interference can also be measured by a lower-thanbreaks (DSBs) that results in gene conversion is frequently accompanied by crossing over (Fogel and Hurst expected incidence of nonparental ditypes (i.e., fourstrand double crossovers) within a single interval. In 1967), whereas an identical DSB is only rarely crossover associated in mitotic cells (Malkova et al. 1996). In budding yeast, mutations such as zip1, msh4, or mlh1, which reduce crossing over, also appear to eliminate the conmitotic cells, crossovers between homologous chromosomes may lead to disadvantageous loss of heterozygosstraints of interference (Ross-Macdonald and Roeder 1994; Sym and Roeder 1994; Khazanehdari and Borts ity; but in meiosis, crossovers play an important role not only in promoting genetic diversity, but in ensuring 2000). In contrast, mus81 or mms4 mutations reduce exchanges without affecting interference (de los Santos proper chromosome segregation (reviewed by Roeder et al. 2003). These observations support the idea that 1997; Zickler and Kleckner 1999; Walker and Hawbudding yeast have two pathways leading to crossovers, ley 2000).but only one of which imposes interference. Intimately connected to the problem of crossover conHow one recombination event might influence antrol at a single locus is the phenomenon of crossover other, at distances of tens of kilobases in yeast and much or chiasma interference, in which the proportion of greater distances in flies and mammals, is not well unclosely spaced crossovers is lower than what would be derstood. Various models have been proposed to exexpected from a random distribution. Interference is plain interference, ranging from those in which the seen in multifactor crosses that produce fewer double constraint is established at the time the DSB is first recombin...

In Escherichia coli, unprotected linear DNA is degraded by exoV activity of the RecBCD nuclease, a protein that plays a central role in the repair of double‐strand breaks. Specific short asymmetric sequences, called chi sites, are hotspots for RecBCD‐promoted recombination and are shown in vitro to attenuate exoV activity. To study RecBCD‐chi site interactions in vivo we used phage lambda's terminase to introduce a site‐specific double‐strand break at lambda's cos site inserted into a plasmid. We show that after terminase has cut cos in vivo, nucleases degrade linearized DNA only from the end that does not have a strong terminase binding site. Linearized cosmid DNA containing chi sites in the proper orientation to the unprotected end is degraded more slowly in rec+ E. coli than is chi‐less DNA. Increased survival of chi‐containing DNA is a result of partial inactivation of exoV activity and is dependent on RecA and SSB proteins. The linearization of chi‐containing DNA molecules leads to RecA‐dependent formation of branched structures which have been proposed as intermediates in the RecBCD pathway of double‐strand break repair.