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. 2013 May;194(1):69–80. doi: 10.1534/genetics.113.150144

Figure 1.

Figure 1

Mechanisms for loss of heterozygosity (LOH) resulting from breakage of a conditional dicentric chromosome. The two chromosome homologs are depicted in G2 with the duplicated chromatids held together at the centromere (shown as ovals or circles). The red and black colors signify polymorphisms that distinguish the two homologs: the black homolog derived from J178-#7-20 and the red homolog from PSL5. Red and black diamonds indicate representative SNPs that distinguish the two homologs: two located in the intercentromic region, one located near the telomere on the same chromosome arm at the conditional centromere, and one located on the opposite chromosome arm. The black homolog carries the GAL-CEN3/URA3 conditional centromere; the conditional centromere is shown as a circle next to the URA3 gene (shown as a green rectangle). When cells are grown on galactose-containing medium, the conditional centromere (white circle) is inactive as a consequence of transcription across the centromere, and the chromosome is functionally monocentric. When the cells are transferred to glucose-containing medium (step 1 in Figure 1, A–C), transcription across the centromere is repressed and the chromosome is functionally dicentric (indicated by black circles) (Hill and Bloom 1989). We show a DSB on only one of the two dicentric chromatids. Four pathways for the repair of the DSB are shown. (A) Reciprocal crossover (RCO). The broken chromosome is repaired by a reciprocal crossover with the homolog (step 2). If the recombined chromosomes segregate as shown by the arrows (step 3), one daughter cell (outlined in black) would be Ura− and the other cell would be Ura+. Only one of the two possible chromosome disjunction patterns is shown; the other pattern does not lead to the markers becoming homozygous. LOH is observed for markers distal to the crossover, but heterozygosity is maintained for the marker on the opposite chromosome arm. (B) Break-induced replication (BIR). In this pathway, one end of the broken black chromatid invades the red chromatid, duplicating all the sequences to the end of the chromatid. The net result of this process is one Ura− (ura3/ura3) cell and one Ura+ (ura3/URA3) cell. The pattern of marker segregation in the Ura− cell is indistinguishable from that shown for the crossover. (C) Conversion without crossover. A DSB occurring near the conditional centromere is processed to yield a conversion event that includes the conditional centromere. If this conversion event is unassociated with a crossover, an interstitial region of LOH would be formed, but heterozygosity would be maintained for the marker near the telomere and the one located on the opposite chromosome arm. (D) Chromosome loss. If the broken chromatid is not repaired, one Ura− monosomic daughter cell and one Ura+ (ura3/URA3) cell would be generated. LOH is observed for all markers.