Molecular and phenotypic analysis of topII-1 and topII-RNAi lines. (A) Schematic progression of interlock formation from early to late zygotene. In the nucleus, numerous entanglements can impede SC polymerization. When two synapsis initiation sites are formed in between homologous chromosomes with another bivalent or chromosome in between, an interlock arises, and synapsis is delayed in that area. dsDNA, double-stranded DNA. (B) AtTOPII gene structure and localization of topII mutations. Boxes in dark blue indicate exons; lines joining the boxes indicate introns; boxes in light blue indicate 5′ and 3′ UTRs. Arrows represent primers used for RT-PCR. (C) Expression of TOPII assessed by RT-PCR of buds of topII-1 and topII-RNAi and their respective WT backgrounds. Exons 19–20 amplify the region before and after the T-DNA insertion in topII-1. Exons 6–10 correspond with the region complementary for the RNAi construct. Exon 20 amplifies the coding region immediately after the T-DNA insertion. GAPDH and Actin2 are constitutive controls. (D) TOPII localization by Western blotting in floral tissue from WT (Ws) and topII-1. Section from ∼150–200 kD. Coomassie staining is shown as a loading control. (E) Representative silique length comparison of Ws, topII-1, and topII-RNAi. (F) Representative pictures of 33-d WT (Ws), topII-1, and topII-RNAi plants. (G) Immunolocalization of TOPII (mouse) and ASY1 in prophase I of WT (Ws), topII-1, and topII-RNAi meiocytes using widefield fluorescence microscopy. Bars, 5 µm.