7An experimental technique called difference topology combined with the mathematics of tangle analysis has been used to unveil the structure of DNA bound by the Mu transpososome. However, difference topology experiments can be difficult and time-consuming.We discuss a modification that greatly simplifies this experimental technique. This simple experiment involves using a topoisomerase to trap DNA crossings bound by a protein complex and then running a gel to determine the crossing number of the knotted product(s). We develop the mathematics needed to analyze the results and apply these results to model the topology of DNA bound by 13S condensin and by the condensin MukB. 8 9 10 11 12 13 KEYWORDS: difference topology experiment, DNA topology, tangle analysis, condensin 14 15 SUMMARY STATEMENT 16 Tangles are used to model protein-DNA complexes: A 3-dimensional ball represents protein while strings embedded in this ball represent 17 protein-bound DNA. We use this simple model to analyze experimental results. 18 19 Proteins bind DNA in many genetic activities such as replication, transcription, packaging, repair, and rearrangement. Understanding the 20 DNA conformation within protein-DNA complexes is useful for modeling and analyzing reactions (Crisona et al., 1999; Harshey and 21