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Molecular and Cellular Biology, September 2000, p. 6308-6316, Vol. 20, No. 17
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Replication of Heterochromatin and Structure of Polytene Chromosomes

Thomas J. Leach,1 Heather L. Chotkowski,1 Michael G. Wotring,1 Robert L. Dilwith,1 and Robert L. Glaser1,2,*

Laboratory of Developmental Genetics, Wadsworth Center, New York State Department of Health,1 and Department of Biomedical Sciences, State University of New York at Albany,2 Albany, New York 12201

Received 21 April 2000/Accepted 30 May 2000

Heterochromatin is characteristically the last portion of the genome to be replicated. In polytene cells, heterochromatic sequences are underreplicated because S phase ends before replication of heterochromatin is completed. Truncated heterochromatic DNAs have been identified in polytene cells of Drosophila and may be the discontinuous molecules that form between fully replicated euchromatic and underreplicated heterochromatic regions of the chromosome. In this report, we characterize the temporal pattern of heterochromatic DNA truncation during development of polytene cells. Underreplication occurred during the first polytene S phase, yet DNA truncation, which was found within heterochromatic sequences of all four Drosophila chromosomes, did not occur until the second polytene S phase. DNA truncation was correlated with underreplication, since increasing the replication of satellite sequences with the cycE1672 mutation caused decreased production of truncated DNAs. Finally, truncation of heterochromatic DNAs was neither quantitatively nor qualitatively affected by modifiers of position effect variegation including the Y chromosome, Su(var)2052, parental origin, or temperature. We propose that heterochromatic satellite sequences present a barrier to DNA replication and that replication forks that transiently stall at such barriers in late S phase of diploid cells are left unresolved in the shortened S phase of polytene cells. DNA truncation then occurs in the second polytene S phase, when new replication forks extend to the position of forks left unresolved in the first polytene S phase.

* Corresponding author. Mailing address: Wadsworth Center, NYS-DOH, P.O. Box 22002, Albany, NY 12201-2002. Phone: (518) 473-4201. Fax: (518) 474-3181. E-mail: glaser{at}wadsworth.org.

Molecular and Cellular Biology, September 2000, p. 6308-6316, Vol. 20, No. 17
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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