[
Trends Genet,
2011]
The unique segregation of homologs, rather than sister chromatids, at the first meiotic division requires the formation of crossovers (COs) between homologs by meiotic recombination in most species. Crossovers do not form at random along chromosomes. Rather, their formation is carefully controlled, both at the stage of formation of DNA double-strand breaks (DSBs) that can initiate COs and during the repair of these DSBs. Here, we review control of DSB formation and two recently recognized controls of DSB repair: CO homeostasis and CO invariance. Crossover homeostasis maintains a constant number of COs per cell when the total number of DSBs in a cell is experimentally or stochastically reduced. Crossover invariance maintains a constant CO density (COs per kb of DNA) across much of the genome despite strong DSB hotspots in some intervals. These recently uncovered phenomena show that CO control is even more complex than previously suspected.
[
Methods Cell Biol,
1995]
The clone-based physical map of the 100-Mb Caenorhabditis elegans genome has evolved over a number of years. Although the detection of clone overlaps and construction of the map have of necessity been carried out centrally, it has been essentially a community project. Without the provision of cloned markers and relevant map information by the C. elegans community as a whole, the map would lack the genetic anchor points and coherent structure that make it a viable entity. Currently, the map consists of 13 mapped contigs totaling in excess of 95 Mb and 2 significant unmapped contigs totaling 1.3 Mb. Telomeric clones are not yet in place. The map carries 600 physically mapped loci, of which 262 have genetic map data. With one exception, the physical extents of the remaining gaps are not known. The exception is the remaining gap on linkage group (LG) II. This has been shown to be bridged by a 225-kb Sse83871 fragment. Because the clones constituting the map are a central resource, there is essentially no necessity for individuals to construct cosmid and yeast artificial chromosome (YAC) libraries. Consequently, such protocols are not included here. Similarly, protocols for clone fingerprinting, which forms the basis of the determination of cosmid overlaps and the mapping of clones received from outside sources and has to be a centralized operation, and YAC linkage are not give here. What follows is essentially a "user's guide" to the physical map. Details of map construction are given where required for interpretation of the map as distributed. The physical mapping has been a collaboration between the MRC Laboratory of Molecular Biology, Cambridge, United Kingdom (now at The Sanger Centre, Cambridge, UK) and Washington University School of Medicine, St. Louis, Missouri. Inquiries regarding map interpretation, information, and materials should be addressed to alan@sanger.ac.uk or rw@nematode.wustl.edu.