by/par Suckjoon Jun and/et John Bechhoefer, Simon Fraser University
Loop formation in long molecules occurs many places in nature, from polymers inside a cell to solutions of carbon nanotubes. When a polymer is flexible, i.e., much longer than its persistence length, entropy favours shorter loops, since fewer configurations need to be searched to find the "target." The loop-formation probability decreases monotonically as the contour length of polymer increases. When a polymer is not much longer than its persistence length ("semiflexible"), the physics is quite different because we now have to take into account the energy cost for bending a stiff polymer, which favours a longer loop. Therefore, there is an optimal loop size of that balances energy and entropy. In this article, we review some theoretical approaches to the study of the static and dynamic properties of polymer loop formation. We then show that simple pictures of loop formation can explain several long-standing observations in DNA replication, quantitatively.