The fission yeast Schizosaccharomyces pombe normally has haploid cells of two 
mating types, which differ at the chromosomal locus mat1. After two consecutive 
asymmetric cell divisions, only one in four 'grand-daughter' cells undergoes a 
'mating-type switch', in which genetic information is transferred to mat1 from 
the mat2-P or mat3-M donor loci. This switching pattern probably results from an 
imprinting event at mat1 that marks one sister chromatid in a strand-specific 
manner, and is related to a site-specific, double-stranded DNA break at mat1. 
Here we show that the genetic imprint is a strand-specific, alkali-labile DNA 
modification at mat1. The DNA break is an artefact, created from the imprint 
during DNA purification. We also propose and test the model that mat1 is 
preferentially replicated by a centromere-distal origin(s), so that the 
strand-specific imprint occurs only during lagging-strand synthesis. Altering 
the origin of replication, by inverting mat1 or introducing an origin of 
replication, affects the imprinting and switching efficiencies in predicted 
ways. Two-dimensional gel analysis confirmed that mat1 is preferentially 
replicated by a centromere-distal origin(s). Thus, the DNA replication machinery 
may confer different developmental potential to sister cells.