Content area
Full Text
ABSTRACT There is evidence accumulating for nonrandom segregation of one or more chromosomes during mitosis in different cell types. We use cell synchrony and two methods to show that all chromatids of budding yeast segregate randomly and that there is no mother-daughter bias with respect to Watson and Crick-containing strands of DNA.
THE immortal strand hypothesis was proposed by J. Cairns as a mechanism to preserve genome integrity during development and was postulated to be especially important for stem cells (Cairns 1975). According to the model, when stem cells undergo asymmetric cell division, one daughter (the self-renewing stem cell) selectively retains the older template DNA strand from each chromosome, avoiding mutations introduced during DNA replication (Cairns 1975; Rando 2007; Tajbakhsh 2008). The model has been tested in a large number of cells from yeast to humans with mixed results and much debate (Neffand Burke 1991; Booth et al. 2002; Merok et al. 2002; Potten et al. 2002; Karpowicz et al. 2005; Conboy et al. 2007; Lansdorp 2007; Rando 2007; Fei and Huttner 2009; Walters 2009; Escobar et al. 2011; Schepers et al. 2011; Yadlapalli et al. 2011). The experiments often utilize halogenated deoxyribonucleotides to label DNA and determine if the label is retained over successive divisions. This protocol was applied to yeast by labeling cells for several generations with 5-bromo-deoxyuridine (BrdU), followed by two rounds of cell division in the presence of unlabeled thymidine to obtain cells in the second mitosis with one unlabeled chromatid and one hemi-labeled chromatid (Neffand Burke 1991). Immunoflourescence was used to follow the fate of the hemi-labeled chromatids after the second mitosis. The immortal strand hypothesis predicts that the oldest (labeled) DNA strands would be segregated to the same daughter; therefore, half of the cells would be labeled and half unlabeled. Random segregation predicts that all of the cells are labeled with each cell containing half as much BrdU. Our results were consistent with random segregation in that all the cells were labeled and the amount of BrdU per cell decreased by half between the first and second division. Sisterchromatid recombination was minimal, and the data could not be explained by nonrandom segregation coupled with sister-chromatid exchange (Neffand Burke 1991).
More recently, a different model for nonrandom chromosome segregation...