Hopefully it will help you! Grindley,1 Katrine L. Whiteson,2 and Phoebe A. Rice2 1Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520-8114; email: nigel.grindley@yale.edu (b) Cuts in one strand of both DNAs. (c) The cut strands cross and join homologous strands, forming the Holliday structure (or Holliday junction). The first plausible model of recombination was proposed by Robin Holliday, and this model (the Holliday model) is still the “prototype model”. This powerpoint animation show how the Holliday Model works. • More modern theories of recombination suggest that 2 Holliday junctions might appear for each crossover. The students were allotted approximately 40 min for completing the model of crossing over based on the time recombination in enzymological terms. instructed to form two models, one cut according to Fig. The Holliday model of DNA crossover. No recombination Recombination Holliday Model. 2g (i) and the other according to Fig. Figure 8-D-2. The physical events of this model, including initiation by nicking, strand exchange to generate symmetric heteroduplex FIG. Use the diagram below if it helps, but make sure to clearly label the diagram. (c) Alternatively, the second DSB end can be captured to form an intermediate that harbors two Holliday junctions (HJ)s, accompanied by gap-filling DNA synthesis and ligation. 5’ _____ 3’ A Holliday … • Gene conversion events can occur depending on the location of the crossover and the material utilized to resolve any gaps. The Holliday model (Holliday, 1964, 1968) was the first widely accepted molecular explanation of the relationship between aberrant segregation and crossing-over. 1. Describe where do these double strand breaks occur in each model. Single-strand scissions are madeat chemically identical sites on the homologous chromatids. Double-stranded cut and 5’ end removal Double-Stranded Break Repair Model Pairing Resolution No recombination Recombination D-loop Formation Strand Synthesis Ligation and Branch Migration Both inner or both outer One inner and one outer. The Holliday model for genetic recombination (3). (4) Both the Holliday and double strand break-repair models for recombination begin with a double strand break. DNA Recombination • Holliday junction model of recombination classically implied a single heteroduplex resolved the DNA during crossing over in meiosis. The Holliday junction is a key intermediate in homologous recombination, a biological process that increases genetic diversity by shifting genes between two chromosomes, as well as site-specific recombination events involving integrases.They are additionally involved in repair of double-strand breaks. Heteroduplex DNAis then formed symmetrically between h and r on both chromatids. ANRV277-BI75-22 ARI 8 May 2006 20:44 Mechanisms of Site-Specific Recombination Nigel D.F. 2g (ii) of the instruction card, resulting in no recombination and recombination of markers flanking the two Holliday junctions, respectively. Finally, wedescribe the Holliday, Meselson-Radding, and double-strand break repair models for recombination and compare and contrast the ability ofthese models to account for the properties of fungal recombination. This model proposed that single strand nicks on the two chromatids involved in the exchange would form heteroduplex regions of the same size. (a) Two homologous DNA molecules line up (e.g., two nonsister chromatids line up during meiosis). (d) Heteroduplex region is formed by branch migration.