DNA replication initiates at multiple sites on plasmid DNA in Xenopus egg extracts

Nucleic Acids Res. 1992 Apr 11;20(7):1457-62. doi: 10.1093/nar/20.7.1457.


Cell-free extracts of Xenopus eggs will replicate plasmid DNA molecules under normal cell cycle control. We have used the neutral/neutral 2-D gel technique to map the sites at which DNA replication initiates in this system. Three different plasmids were studied: one containing the Xenopus rDNA repeat, one containing single copy Xenopus genomic DNA, and another containing the yeast 2 microns replication origin. 2-D gel profiles show that many potential sites of initiation are present on each plasmid, and are randomly situated at the level of resolution of this technique (500-1000 bp). Despite the abundance of sites capable of supporting the initiation of replication, pulse-chase experiments suggest that only a single randomly situated initiation event occurs on each DNA molecule. Once initiation has taken place, conventional replication forks appear to move away from this site at a rate of about 10nt/second, similar to the rate observed in vivo.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Cell Extracts / genetics
  • DNA Replication / genetics*
  • DNA, Ribosomal / genetics
  • Electrophoresis, Gel, Two-Dimensional
  • Ovum / metabolism*
  • Plasmids / genetics*
  • Xenopus laevis


  • Cell Extracts
  • DNA, Ribosomal