Single-molecule measurements of double-stranded DNA condensation

Electrostatically driven double-stranded DNA (dsDNA) condensation is critical in regulating many biological processes, including bacteriophage and virus replication and the packaging of chromosomal DNA in sperm heads. Here, we review single-molecule measurements of dsDNA condensed by cationic proteins, polypeptides, and small multivalent cations. Optical tweezers (OT) measurements of dsDNA collapsed by cationic condensing agents reveal a critical condensing force unique to each condensing agent that is tunable with condensing agent concentration and ionic strength. DNA globules visualized via atomic force microscopy, transmission electron microscopy, and cryoelectron microscopy reveal condensed dsDNA adopting several conformations including highly ordered toroids with a measurable central hole and, more recently, the maximally dense, yarn-ball-like structures observed with dsDNA condensed by the HIV-1 nucleocapsid protein. The combination of these approaches provides multifaceted insight into the shape and size of electrostatically condensed dsDNA globules and the kinetics of their formation and dissolution. We also review the physics of dsDNA condensation, including recent studies that show dsDNA globule size is tunable with ionic strength. Overall, this review provides important insights into understanding dsDNA condensate-regulated biological processes, as well as potential uses for gene delivery.