In interphase nuclei and metaphase chromosomes, eukaryotic chromosomal DNA is arranged in negatively supercoiled loops whose length averages several tens of kilobases (kb). Supercoiling indicates that the loops are tenaciously bound at both ends through the periodic attachment of DNA to a non-histone protein structural support, termed the nuclear matrix, skeleton or cage in interphase nuclei and the scaffold in metaphase chromosomes1-16. The looped organization has been envisaged to be important in DNA replication 2,11,15,17-25 and transcription26-28. We report here a close relationship between average loop length and replicon size in different animal and plant species. Autoradiographic experiments support the hypothesis that DNA synthesis occurs at fixed sites on the nuclear matrix15,20. We also describe a modification of the looped organization occurring during early embryonic development in Xenopus laevis.
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