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In molecular biology, strand displacement frequently denotes a process mediated by enzymes such as polymerases, but the reaction as defined above is guided by the biophysics of DNA andoccurs independently of enzymes.  Enzyme-free strand displacement and branch migration have been studied since the 1970s, but have only been applied to DNA nanotechnology within the past decade.
DNAstrand displacement
Stranddisplacement is theprocess through which two strands withpartial or fullcomplementarity hybridize to each other, displacing one or more pre-hybridized strands in the process. Strand displacement can be initiated at complementary single-stranded domains (referred to as toeholds) and progresses through a branch migration process that resembles a random walk. By varying the strength (length and sequence composition) of toeholds, the rate of strand-displacement reactions can be quantitatively controlled over a factor of 106. Moreimportantly, this feature allows engineering control over the kinetics of synthetic DNA devices.
[[Image:wiki-2'_clip_image002_0001.jpg|thumb|550px|center|Fig 1.<br>]]
Purpose
We use aDNA sequence to connect the DNA origami which is loaded with functional molecules, such us antitumor drugs, with the Au nanoparticles. In thiscase, the function of this DNA sequence is to help releasing the drugs at certain temperature and the drugs will never go back to the nanoparticles. For another, we hope that the deviceenables distinct drugsreleaesd under different thermal conditions in batches.
Design
Irreversible release
To make the process of releasing drugs irreversible, we need to design a kind of DNA sequence which will be inactive at the single state, while active when it is complementary.
We find out the hairpin structure which can exactly accordwith our idea.
Because of the role of the toehold in initiating strand displacementreactions, strands can be rendered effectively inactive if thetoehold domain is made inaccessible by toehold sequestering.Toehold sequestering can be achieved in a number of ways, two most common of which are hybridization of the toehold to acomplementarydomain and isolation of the toehold in ashort hairpin structure where helix formation is difficult. Programmed sequestering and subsequentexposure of toehold domains allows precise control of order andtiming over the reactions and has been used in conjunctions withtoehold-mediated stranddisplacement to construct molecularmotors,polymerization reactions, catalytic reactions,andlogic gates.
[[Image:wiki-2'_clip_image005_0000.jpg|thumb|550px|left|Fig 1.<br>]]


==Synthesis of AU-DNA-CY3==
==Synthesis of AU-DNA-CY3==

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Strand Replacement Reaction

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Synthesis of AU-DNA-CY3

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Fig 1. Lasers trigger the photothermal effect and the local temperature raises above the melting temperature of the DNA duplex, which allows the nonthiolated strand to dissociate into the surrounding medium while its complementary remains attached to the gold nanoparticle.

For the purpose of assembling the origami-GNPs complex,i.e. attaching GNPs to DNA origami as illustrated in Figure 1, we use thiol-modified oligonucleotides (short synthetic DNA sequences), which can be loaded onto the surface of GNPs to combine GNPs and DNA. And we choose Cy3 fluorescent dye to target the DNA strand for the detection of combination. GNPs are fixed onto the DNA origami by linking them to staple strands whose 5’end are modified with lipoic acid. Upon hybridization between DNA tails on DNA origami and single staple strands on GNPs, GNPs are attached to the DNA origami, resulting in the formation of origami-GNPs complex. DNA origami is used for in vivo delivery of chemotherapeutic drugs in our project. Originally, the photothermal property of GNPs was used in biologically relevant studies to destroy cancer cells, while, in our project, it has been harnessed as a means to optically elicit the release of drugs encapsulated in DNA origami.

DNA Origami

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