Illustrations of single-molecule manipulation techniques. (a) AFM. The molecule is held by the tip and the substrate surface. The force on the attached molecule is determined based on the deflection of the cantilever. Reproduced from Ref. [21], Copyright © 2010 American Physical Society. (b) Optical tweezers. One end of a DNA molecule is attached to a bead trapped by a laser beam, while the other end is attached to a DNA-virus capsid complex on a second bead, held by a micropipette tip. Reproduced from Ref. [8], Copyright © 2001 Nature Publishing Group. (c) Magnetic tweezers. A force is exerted on the molecule by an attached super-paramagnetic bead in a magnetic field. The molecule can be twisted as well as stretched by the applied field. Reproduced from Ref. [22], Copyright © 2006 American Physical Society.