invites you to a


to be presented by

Professor De Witt Sumners

Florida State University


Lecture I:  Monday 31 March, 2008 at 15:30

 Benjamin Auditorium (Sego 1) Sego Architecture Building

Introduction to DNA Topology


Cellular DNA is a long, thread-like molecule with remarkably complex topology.  Enzymes that manipulate the geometry and topology of cellular DNA perform many important cellular processes (including segregation of daughter chromosomes, gene regulation, DNA repair, and generation of antibody diversity).  Some enzymes pass DNA through itself via enzyme-bridged transient breaks in the DNA; other enzymes break the DNA apart and reconnect it to different ends.  In the topological approach to enzymology, circular DNA is incubated with an enzyme, producing an enzyme signature in the form of DNA knots and links. By observing the changes in DNA geometry (supercoiling) and topology (knotting and linking) due to enzyme action, the enzyme binding and mechanism can often be characterized. This talk is intended for a general malthematical audience, and will discuss topological models for DNA strand passage and exchange, including the analysis of topoisomerase experiments on circular DNA using knot theory.

Lecture II:  Wednesday, 2 April, 2008  at 15:30

Butler Auditorium, Samuel Neaman Institution 

The Tangle Model for DNA Site-Specific Recombination


DNA site-specific recombination is a vital step in cellular metabolism, essential in the life cycle of viruses and gene regulation.  In site-specific recombination, a recombinase enzyme binds to two duplex DNA segments, makes two double-strand breaks, and reconnects the DNA to different ends, thus changing the genotype of the organism.  Topological enzymology experiments place both recombination sites on the same unknotted substrate DNA circle, and recombination on that substrate produces a product spectrum of enzyme-specific family of DNA knots and links.  The tangle model uses the methods of geometric topology to compute enzyme binding and mechanism from the observed family of DNA knots and links.



Lecture III:  Thursday, 3 April, 2008  at 15:30

Benjamin Auditorium (Sego 1) Sego Architecture Building 

Random Knotting and Viral DNA Packing


This lecture will present the proof of the Frisch-Wasserman-Delbruck Conjecture:  As the length of random curves in 3-space goes to infinity, the knotting probability goes to one.  As application, we will use DNA knotting as an assay for packing geometry of DNA in viral capsidsBacteriophages are viruses that infect bacteria.  They pack their double-stranded DNA genomes to near-crystalline density in viral capsids and achieve one of the highest levels of DNA condensation found in nature. Despite numerous studies some essential properties of the packaging geometry of the DNA inside the phage capsid are still unknown.  Although viral DNA is linear double-stranded with sticky ends, the linear viral DNA quickly becomes cyclic when removed from the capsid, and for some viral DNA the observed knot probability is an astounding 95%.  This talk will discuss comparison of the observed viral knot spectrum with the simulated knot spectrum, concluding that the packing geometry of the DNA inside the capsid is non-random and writhe-directed.