《Nature》封面故事：DNA也可以让“蒙娜丽莎”微笑（华中科技大学栗茂腾）：《蒙娜丽莎的微笑》是一幅享有盛誉的肖像画杰作，是意大利文艺复兴时代著名画家达·芬奇的最高艺术成就。近日，生物学家利用DNA折纸技术画出了世界上最小的该著名画作。

DNA折纸（DNA origami）是加州理工大学Paul Rothemund在2006年开发的把长链DNA折叠成规定形状的技术，该技术极大地拓展了纳米科技领域。在最新Nature封面文章“Fractal assembly of micrometre-scale DNA origami arrays with arbitrary patterns”中，钱璐璐团队根据数学Fractals概念研发了新的DNA折纸技术，并折出了世界上最小幅的《蒙娜丽莎的微笑》。这是该团队近期发表的第二篇DNA折纸的文章，此前该团队还开发出了一种全自动的DNA分子机器，可在纳米尺度上执行任务，相关研究发表在9月15日出版的《科学》杂志上（A cargo-sorting DNA robot）。

Nature期刊发表的这篇极具艺术美感的封面文章中，研究人员利用fractal assembly的方法以最小化拼图的边缘特征，先用64只试管合成64片‘拼图’，把每只试管编号以确保它们在整张图上的位置，先依次混合4只邻近的试管，得到16个尺寸为2×2的方块，再将它们四四结合，得到4个4×4的大方块，最后将这4个拼起来凑出整张图画（部分文字参考生物通和caltech网站）。

看到DNA折纸研究的最新进展备受鼓舞，本人2015年开始参与指导我校BIOMOD（InternationalBio-molecular Design Competition）团队进行DNA折纸相关研究，2015年提出“调控生物因子平衡的DNA纳米装置”概念，结合“DNA折纸术”这一先进的纳米技术手段，应用原子力显微镜成功解析了新设计的分子装置的准确结构，并通过凝胶电泳、荧光共振能量转移和凝血酶原时间测试等各类实验方法成功验证该装置快速响应调控生物因子平衡的各项功能。该项目参加在哈佛大学举办的国际生物分子设计大赛，一举夺得金奖；2016年，相关研究结果以A Quick-responsive DNA Nanotechnology Device for Bio-molecular Homeostasis Regulation发表在Scientific Reports期刊上。

2017年9月14日，另外一个以DNA折纸为基础 研究结果以Full Paper形式发表在Small期刊上，论文标题为A DNA Origami Mechanical Device for the Regulation of Microcosmic Structural Rigidity（可用于调控微观结构刚性的DNA折纸纳米器件）。这篇论文主要通过“DNA折纸术”这一技术手段，构建了由内部中心轴和外部滑动管组成的多聚体DNA纳米结构，单体外管在DNA动力链的触发诱导下能够沿着内部中心轴滑动，从而实现该纳米器件在微观尺度的柔性/刚性状态之间转换。该纳米结构的形态转换，还被证明能够有效调节其所附着物质的微观结构刚性。由于微观结构刚性对于细胞增殖和功能十分重要，此研究结果为使用DNA纳米结构调节细胞刚性从而改变其生理状态提供了理论与实践基础。该论文研究内容延续自BIOMOD HUST-China团队2016年参与在加州大学旧金山分校举办的BIOMOD国际生物分子设计大赛并夺得银奖的参赛项目。

英文原文介绍如下（http://www.admissions.caltech.edu/content/worlds-smallest-mona-lisa）：

In 2006, Caltech's Paul Rothemund (BS '94) —now research professor of bioengineering, computing and mathematical sciences, and computation and neural systems—developed a method to fold a long strand of DNA into a prescribed shape. The technique, dubbed DNA origami, enabled scientists to create self-assembling DNA structures that could carry any specified pattern, such asa 100-nanometer-wide smiley face.

DNA origami revolutionized the field of nanotechnology, opening up possibilities of building tiny molecular devices or "smart" programmable materials. However, some of these applications require much larger DNA origami structures. Now, scientists in the laboratory of Lulu Qian, assistant professor of bioengineering at Caltech, have developed an inexpensive method by which DNA origami self-assembles into large arrays with entirely customizable patterns, creating a sort of canvas that can display any image. To demonstrate this, the team created the world's smallest recreation of  Leonardo da Vinci's Mona Lisa—out of  DNA. The work isdescribed in a paper appearing in the December 7 issue of the journal Nature.

While DNA is perhaps best known for encoding the genetic information of living things, the molecule is also an excellent chemical building block. A single-stranded DNA molecule is composed of smaller molecules called nucleotides—abbreviated A, T,C, and G—arranged in a string, or sequence. The nucleotides in asingle-stranded DNA molecule can bond with those of another single strand toform double-stranded DNA, but the nucleotides bind only in very specific ways: an A nucleotide with a T or a C nucleotide with a G. These strict base-pairing "rules" make it possible to design DNA origami.

To make asingle square of DNA origami, one just needs a long single strand of DNA and many shorter single strands—called staples—designed to bind to multiple designated places on the long strand. When the short staples and the long strand are combined in a test tube, the staples pull regions of the long strand together, causing it to fold over itself into the desired shape. A large DNA can vas is assembled out of many smaller square origami tiles, like putting together a puzzle. Molecules can be selectively attached to the staples inorder to create a raised pattern that can be seen using atomic forcemicroscopy.

The Caltech team developed software that can take an image such as the MonaLisa, divide it up into small square sections, and determine the DNAsequences needed to make up those squares. Next, their challenge was to get those sections to self-assemble into a superstructure that recreates the Mona Lisa.  "We could make eachtile with unique edge staples so that they could only bind to certain othertiles and self-assemble into a unique position in the superstructure," explains Grigory Tikhomirov, senior postdoctoral scholar and the paper's leadauthor, " but then we would have to have hundreds of unique edges, which would be not only very difficult to design but also extremely expensive to synthesize. We wanted to only use a small number of different edge staples but still get all the tiles in the right places."

The key to doing this was to assemble the tiles in stages, like assembling small regions of a puzzle and then assembling those to make larger regions before finally putting the larger regions together to make the completed puzzle. Each minipuzzle utilizes the same four edges, but because these puzzles are assembled separately, there is no risk, for example, of a corner tile attaching in the wrong corner. The team has called the method "fractal assembly" because the same set of assembly rules is applied at different scales.

"Once we have synthesized each individual tile, we place each one into its own test tube for a total of 64 tubes," says Philip Petersen, a graduate student andco-first author on the paper. "We know exactly which tiles are in which tubes, so we know how to combine them to assemble the final product. First, we combine the contents of four particular tubes together until we get 16 two-by-two squares. Then those are combined in a certain way to get four tubes each with a four-by-four square. And then the final four tubes are combined tocreate one large, eight-by-eight square composed of 64 tiles. We design the edges of each tile so that we know exactly how they will combine."

The Qian team's final structure was 64 times larger than the original DNA origami structure designed by Rothemund in 2006. Remarkably, thanks to the recycling of the same edge interactions, the number of different DNA strands required forthe assembly of this DNA superstructure was about the same as for Rothemund's original origami. This should make the new method similarly affordable, according to Qian.

"The hierarchical nature of our approach allows using only a small and constant set of unique building blocks, in this case DNA strands with unique sequences, to build structures with increasing sizes and, in principle, an unlimited number of different paintings," says Tikhomirov. " This economical approach of building more with less is similar to how our bodies are built.  All our cells have the same genome and are built using the same set of building blocks, such as amino acids, carbohydrates, and lipids.  However, via varying gene expression, each cell uses the same building blocks to build different machinery, for example, muscle cells and cells in the retina."

The team also created software to enable scientists everywhere to create DNA nanostructures using fractal assembly. " To make our technique readily accessible to other researchers who are interested in exploring applications using micrometer-scalefl at DNA nanostructures, we developed an online software tool that converts the user's desired image to DNA strands and wet-lab protocols," says Qian." The protocol can be directly read by a liquid-handling robot to automatically mix the DNA strands together. The DNA nanostructure can beassembled effortlessly."

Using this online software tool and automatic liquid-handling techniques, several other patterns were designed and assembled from DNA strands, including a life-sizedportrait of a bacterium and a bacterium-sized portrait of a rooster. "Other researchers have previously worked on attaching diverse molecules such as polymers, proteins, and nanoparticles to much smaller DNA can vases for the purpose of building electronic circuits with tiny features, fabricating advanced materials, or studying the interactions between chemicals or biomolecules," says Petersen. "Our work gives them an even larger canvas to draw upon."

The paper istitled "Fractal assembly of micrometre-scale DNA origamiarrays with arbitrary patterns." The work was funded by the Burroughs Wellcome Fund, the National Institutes of Health's National Research Service Award,  and the National Science Foundation's Expeditions in Computing program.