We compared the structure of discussions in a middle school mathematics classroom before (Year 1) and after (Year 2) teacher participation in professional development activities aimed at enhancing students' participation and the co-construction of mathematical ideas. Changes in the role of teacher and student were accompanied by identifiable changes in discourse structure. In particular, while traditional initiation-response-evaluation (IRE) patterns continued throughout, the de-centering of the teacher's authority led to a reduction in IRE occurrences and increases in student-directed initiation-demonstration-evaluation (IDE) patterns. Analyses conducted by human coders were corroborated by computer-based motif analyses that identified flexible patterns using probabilistic, data-mining methods, while also predicting some novel discourse structures. Appended are: (1) Event Codes Used, with Examples from Transcripts; (2) Example Section from Year 1, Lesson 1, of the Stream of Codes Obtained from the Four Lesson Transcripts Used for Both Human and Computer-Based Pattern Finding; and (3) Motif Example. (Contains 6 tables and 2 figures.)