  Loop formation in long molecules occurs many places in nature, from solutions
of carbon nanotubes to polymers inside a cell. In this article, we review
theoretical studies of the static and dynamic properties of polymer loops. For
example, long polymers must search many configurations to find a "target"
binding site, while short polymers are stiff and resist bending. In between,
there is an optimal loop size, which balances the entropy of long loops against
the energetic cost of short loops. We show that such simple pictures of loop
formation can explain several long-standing observations in DNA replication,
quantitatively.
