Aberrant replication timing induces defective chromosome condensation in Drosophila ORC2 mutants.
BACKGROUND
The accurate duplication and packaging of the genome is an absolute prerequisite to the segregation of chromosomes in mitosis. To understand the process of cell-cycle chromosome dynamics further, we have performed the first detailed characterization of a mutation affecting mitotic chromosome condensation in a metazoan. Our combined genetic and cytological approaches in Drosophila complement and extend existing work employing yeast genetics and Xenopus in vitro extract systems to characterize higher-order chromosome structure and function.


RESULTS
Two alleles of the ORC2 gene were found to cause death late in larval development, with defects in cell-cycle progression (delays in S-phase entry and metaphase exit) and chromosome condensation in mitosis. During S-phase progression in wild-type cells, euchromatin replicates early and heterochromatin replicates late. Both alleles disrupted the normal pattern of chromosomal replication, with some euchromatic regions replicating even later than heterochromatin. Mitotic chromosomes were irregularly condensed, with the abnormally late replicating regions of euchromatin exhibiting the greatest problems in mitotic condensation.


CONCLUSIONS
The results not only reveal novel functions for ORC2 in chromosome architecture in metazoans, they also suggest that the correct timing of DNA replication may be essential for the assembly of chromatin that is fully competent to undergo mitotic condensation.
Loupart
M L
ML
University of Edinburgh, Wellcome Trust Centre for Cell Biology, Institute of Cell and Molecular Biology, Michael Swann Building, King's Buildings, Mayfield Road, Edinburgh EH9 3JR, UK.
Krause
S A
SA
Heck
M S
MS
D000818:Animals; D001973:Bromodeoxyuridine / Q000494:pharmacology; D002453:Cell Cycle / Q000502:physiology*; D002478:Cells, Cultured; D002875:Chromosomes / Q000378:metabolism*; D004247:DNA / Q000378:metabolism; D004261:DNA Replication / Q000187:drug effects; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D029721:Drosophila Proteins; D004331:Drosophila melanogaster / Q000196:embryology / Q000235:genetics / Q000502:physiology*; D005260:Female; D017404:In Situ Hybridization, Fluorescence; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D009154:Mutation; D051738:Origin Recognition Complex; D010641:Phenotype; D013997:Time Factors
D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; C497216:Orc2 protein, Drosophila; D051738:Origin Recognition Complex; D004247:DNA; D001973:Bromodeoxyuridine