R-loop mapping of exons and introns of the adenovirus hexon gene. (A) Arrangement of exons (blue segments) and introns (lettered A through C) along a portion of adenovirus DNA. The location of an EcoRI restriction fragment is also indicated. (B) Electron micrograph of an R-loop between the EcoRI-A DNA fragment and the mRNA derived from that region of the genome. (C) Interpretive diagram of the R-loop structure. DNA is shown in blue and RNA is shown in red.