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J. Cell Biol.,
Volume 141, Number 1, April 6, 1998 5-20
* Graduate Group in Biophysics and The dynamics by which homologous chromosomes pair is currently unknown. Here, we use fluorescence in situ hybridization in combination with
three-dimensional optical microscopy to show that homologous pairing of the somatic chromosome arm 2L
in Drosophila occurs by independent initiation of pairing at discrete loci rather than by a processive zippering
of sites along the length of chromosome. By evaluating
the pairing frequencies of 11 loci on chromosome arm
2L over several timepoints during Drosophila embryonic development, we show that all 11 loci are paired
very early in Drosophila development, within 13 h after
egg deposition. To elucidate whether such pairing occurs by directed or undirected motion, we analyzed the
pairing kinetics of histone loci during nuclear cycle 14. By measuring changes of nuclear length and correlating
these changes with progression of time during cycle 14, we were able to express the pairing frequency and distance between homologous loci as a function of time.
Comparing the experimentally determined dynamics of
pairing to simulations based on previously proposed
models of pairing motion, we show that the observed
pairing kinetics are most consistent with a constrained
random walk model and not consistent with a directed motion model. Thus, we conclude that simple random
contacts through diffusion could suffice to allow pairing
of homologous sites.
Department of Biochemistry and Biophysics, University of California, San Francisco,
California 94143-0554; and The Howard Hughes Medical Institute, San Franciso, California 94143
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