Changes in the configuration of the structural DNA loops as a function of DNA replication. A) Structural DNA loop organization of the 162 kbp region containing the rat albumin locus in undisturbed, control G0 hepatocytes. The letters (a-o) indicate the position of the corresponding target sequences relative to the NM. Color lines indicate the position of the corresponding named gene. The small dashed lines indicate projected loop regions outside of the region studied. The black ovals within the NM correspond to putative LARs. B) Average configuration of the structural DNA loops in the same region at the peak of DNA synthesis (24 h after partial hepatectomy) deduced from the results in Tables 2 and 3. Dashed lines indicate the segment of the corresponding gene possibly awaiting replication. Bold-color lines indicate the daughter duplexes containing one parental strand and one newly made strand, that are extruded in loops as the parental duplex slides through the fixed replication sites on the NM (parental loops shrink whereas daughter loops grow). Letters (b...) indicate target sequences to be replicated. Letters with apostrophe (a'...) indicate target sequences that have been replicated already. The double-headed arrows in the NM correspond to active replication forks derived from putative origins of replication located close to the LARs. In this representation the early-replicating LARs (black ovals) become attached to the NM immediately after replication, thus producing tiny but transient duplicate daughter loops that should disappear once the whole region has been duplicated.