These four strands associate into a DNA four-arm junction because this structure maximizes the number of correct
for a more realistic model of the four-arm junction showing its
This double-crossover (DX)
consists of five
single strands that form two
domains, on the top and the bottom in this image. There are two crossover points where the strands cross from one domain into the other.
Properties of nucleic acids
Nanotechnology is often defined as the study of materials and devices with features on a scale below 100
nanometers. DNA nanotechnology, specifically, is an example of
molecular self-assembly, in which molecular components spontaneously organize into stable structures; the particular form of these structures is induced by the physical and chemical properties of the components selected by the designers.
 In DNA nanotechnology, the component materials are strands of nucleic acids such as DNA; these strands are often synthetic and are almost always used outside the context of a living cell. DNA is well-suited to nanoscale construction because the binding between two nucleic acid strands depends on simple
base pairing rules which are well understood, and form the specific nanoscale structure of the
nucleic acid double helix. These qualities make the assembly of nucleic acid structures easy to control through
nucleic acid design. This property is absent in other materials used in nanotechnology, including
proteins, for which
protein design is very difficult, and
nanoparticles, which lack the capability for specific assembly on their own.
structure of a nucleic acid molecule consists of a sequence of
nucleotides distinguished by which
nucleobase they contain. In DNA, the four bases present are
guanine (G), and
thymine (T). Nucleic acids have the property that two molecules will only bind to each other to form a double helix if the two sequences are
complementary, meaning that they form matching sequences of base pairs, with A only binding to T, and C only to G.
 Because the formation of correctly matched base pairs is
energetically favorable, nucleic acid strands are expected in most cases to bind to each other in the conformation that maximizes the number of correctly paired bases. The sequences of bases in a system of strands thus determine the pattern of binding and the overall structure in an easily controllable way. In DNA nanotechnology, the base sequences of strands are rationally designed by researchers so that the base pairing interactions cause the strands to assemble in the desired conformation.
DNA is the dominant material used, structures incorporating other nucleic acids such as
peptide nucleic acid (PNA) have also been constructed.
DNA nanotechnology is sometimes divided into two overlapping subfields: structural DNA nanotechnology and dynamic DNA nanotechnology. Structural DNA nanotechnology, sometimes abbreviated as SDN, focuses on synthesizing and characterizing nucleic acid complexes and materials that assemble into a static,
equilibrium end state. On the other hand, dynamic DNA nanotechnology focuses on complexes with useful non-equilibrium behavior such as the ability to reconfigure based on a chemical or physical stimulus. Some complexes, such as nucleic acid nanomechanical devices, combine features of both the structural and dynamic subfields.
The complexes constructed in structural DNA nanotechnology use topologically branched nucleic acid structures containing junctions. (In contrast, most biological DNA exists as an unbranched
double helix.) One of the simplest branched structures is a four-arm junction that consists of four individual DNA strands, portions of which are complementary in a specific pattern. Unlike in natural
Holliday junctions, each arm in the artificial immobile four-arm junction has a different
base sequence, causing the junction point to be fixed at a certain position. Multiple junctions can be combined in the same complex, such as in the widely used double-crossover (DX)
structural motif, which contains two parallel double helical domains with individual strands crossing between the domains at two crossover points. Each crossover point is, topologically, a four-arm junction, but is constrained to one orientation, in contrast to the flexible single four-arm junction, providing a rigidity that makes the DX motif suitable as a structural building block for larger DNA complexes.
Dynamic DNA nanotechnology uses a mechanism called
toehold-mediated strand displacement to allow the nucleic acid complexes to reconfigure in response to the addition of a new nucleic acid strand. In this reaction, the incoming strand binds to a
single-stranded toehold region of a double-stranded complex, and then displaces one of the strands bound in the original complex through a
branch migration process. The overall effect is that one of the strands in the complex is replaced with another one.
 In addition, reconfigurable structures and devices can be made using functional nucleic acids such as
ribozymes, which can perform chemical reactions, and
aptamers, which can bind to specific proteins or small molecules.